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This is the farthest extreme of a development of the head, which, beginning with such forms as pellucida, runs through galeata, apicata, berolinensis, vitrea, kahlenbergensis and cederströmii to the present species, where it reaches truly enormous proportions. The meaning of such a character I am not able to imagine. The expansion of the head is a thin and flexible plate, affording lodgment to no organs, and seems an utterly useless encumbrance.

In Geneva lake, Wisconsin, the most abundant entomostracan in October, was an extremely variable Daphnia approaching hyalina on the one hand and retrocurva on the other, but still separable from both. It is evident that this group of helmeted Daphnias is still in process of active evolution, and it is possible that there are no actual breaks anywhere along the line from hyalina to retrocurva, although in the former the head may be scarcely larger than in Daphia pulex, while in the latter it is often more than half as large as the body.

Comparing the Daphnias of Lake Michigan with those of Geneva lake, Wis. (nine miles long and twenty-three fathoms in depth), those of Long lake, Ills. (one and a half miles long and six fathoms deep), and those of other still smaller lakes of that region, a curious progressive predominance of the large-helmeted forms is very evident in passing from larger to smaller lakes. If we extend the comparison further, and include the other entomostraca, and the swamps and smaller ponds as well, we shall be struck by the inferior development of the entomostraca of the larger bodies of water, in numbers, in size and robustness, and in reproductive power. Their smaller numbers and size are doubtless due to the relative scarcity of food. The system of aquatic animal life rests essentially upon the vegetable world, although perhaps less strictly than does the terrestrial system; and in a large and deep lake vegetation is much less abundant than in a narrower and shallower one, not only relatively to the amount of water but also to the area of the bottom. (In all the lakes which I have dredged, life of all sorts was much more scanty in the interior deeper portions than along the margins.) From this deficiency of plant life results a deficiency of food for entomostraca, whether of Algæ, of Protozoa or of higher forms, and hence, of course, a smaller number of the entomostraca themselves, with more slender bodies suitable for more rapid locomotion.

The difference of reproductive energy, as shown by the much smaller egg-masses borne by the lacustrine species, depends upon the vastly greater destruction to which the paludinal crustacea are subjected. Many of the latter occupy waters liable to be exhausted by drought, with a consequent enormous waste of entomostracan life. The opportunity for reproduction is here greatly limited-in some situations to early spring alone-and the chances for destruction of the summer eggs in the dry and often dusty soil are so numerous that only the most prolific species can maintain themselves under such conditions.

Further, the marshes and shallower lakes are the favorite breeding grounds of fishes, which migrate to them in spawning time, if possible, and it is from the entomostraca found here that most young fishes get their earliest food supplies—a danger from which the deep-water species are measurably free. Not only is a high reproductive power therefore rendered unnecessary among the latter by their freedom from many dangers to which the shallow-water species are exposed, but in view of the relatively small amount of food available for them, a high rate of multiplication would be a positive injury, and could result only in wholesale starvation.

EXPLANATION OF PLATE IX.

FIG. 1.-Osphranticum labronectum, second maxilliped, X 123.

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2.

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mandible and palpus, × 75.

3. Leptodora hyalina, (After Weissman).
4.-Osphranticum labronectum, ♂, antennæ, X 38.

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clubs, X 120.

6. Cyclops insectus,, x 38.

♂, part of left antenna, showing olfactory

7.-Osphranticum labronectum, leg of fourth pair.

8.-Epischura lacustris, ♂, × 42. (Some of the basal segments of the an tenna are concealed. The segmentation of the cephalothorax is incompletely shown. See text.)

9.-Osphranticum labronectum, antennula.

10.-Cyclops thomasi, leg of fifth pair.

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NEW GENERA AND SPECIES OF COPEPODA.

Genus OSPHRANTICUM, gen. nov.

(Plate vin Figs. 24, 28 and 29; Plate Ix Figs. 1, 2, 4, 5, 7 and 9.)

This genus is similar to Diaptomus in general appearance, but differs especially in the structure of the fifth pair of legs of male and female.

The antennæ are 23-jointed, the right of the male geniculate between the 18th and 19th. Joints 13-18 are dilated on this antenna, and the 19th and 20th are united, reducing the number to 22.

The antennules and mouth parts have the same general structure as in Diaptomus, but the former are unusually large. The legs of the first pair have both rami 3jointed. In the male both legs of the fifth pair are bi-ramose and armed with plumose cilia (Pl. VIII Fig. 29). The inner ramus of each is 3-jointed and unmodified. In the outer ramus of the left leg the second and third joints are consolidated, and bear three plumose terminal setæ, a strong spine on the outer margin and a pubescent tubercle at the inner base. The outer ramus of the right leg is 3-jointed, bears two setæ at tip aud is unarmed within.

In the femile the legs of the fifth pair are nearly alike and only the third joint of the outer ramus is modified (compare Fig. 28 Pl. vIII, with Fig. 7 Pl. 1x).

1. Osphranticum labronectum, sp. nov.

The cephalothorax is oval, symmetrical, and composed of six segments regularly decreasing in length from before backwards. The head is not distinct from the first thoracic segment, and the posterior angles of the cephalothorax are evenly rounded. The abdomen is cylindrical and unarmed; of five segments in the male, four in the female. The antennæ scarcely surpass the cephalothorax, and are very richly sup plied with olfactory clubs. The second joint is nearly as long as the three following. The set of the antennules reach to the 20th joint of the antennæ. The egg-sac in the female is unusually large, obovate, widest posteriorly, flattened vertically, and extending to the tips of the setæ. The latter are five in number to each ramus, the fourth from the outside being much the longest.

This species is readily distinguished in life by the short and compact thorax (its depth being contained but two and a half times in its length), and by its steady movement in the water, as it does not commonly swim with the jerking motion of Diaptomus. Those taken at Normal were pale brown in color, without markings. Found in a wayside pool at Normal, Ill., in February, 1877, and in swamps in Iroquois county, in the same month of 1882. The females of the latter lot were bearing eggs.

Genus DIAPTOMUS.

1. Diaptomus sicilis, sp. nov. (Pl. vII Figs. 9 and 20.)

This is the American representative of Diaptomus gracilis of Europe. It differs from that species as described by Sars, as follows: The antepenultimate joint of the right male antenna is not armed with a hook, but with a long and slender spinelike process, nearly equaling the following joint. The last joint of the fifth pair of legs of the female is not distinct, nor indeed at all distinguishable, and its terminal spines reach less than half the distance to the tip of the claw of the penultimate joint. The inner ramus of this leg is much longer than the basal joint of the outer ramus, and is not 2-jointed in any specimen which I have examined. The terminal claw of the right foot of the male is regularly curved from base to apex.

The species is usually colorless, although I have seen occasionally individuals of

a uniform crimson. Equally conspicuous differences are apparent on comparison of the legs of the fifth pair of the male with the descriptions and figures of Gruber' (see Fig. 9).

The thoracic segments corresponding to the two last pairs of legs are not distinct, but the head is divided into an anterior and a posterior part by an evident constriction and an incomplete suture. The body is .065 in. long (without caudal setæ) by one-fourth that depth.

This species has a special economical value as constituting, with one species of Cyclops, hereafter to be described, almost the entire first food of the whitefish, I have not found it anywhere outside of Lake Michigan, but there it occurs in im mense numbers; sometimes being the most abundant species appearing in the net. A similar but not identical form occurs in the small and shallow lakes of Northern Illinois and Indiana. The latter is possibly the D. pallidus of Herrick,2 although neither his description nor figures are really specific.

2. Diapiomus leptopus, sp. nov. (Pl. vIII Figs. 17-19.)

This species resembles the foregoing in general appearance, but may be easily distinguished by the relative robustness of the antennæ and the shortness of their hairs and spines, by the width and shortness of the rami of the furca (the width being a little more than two-thirds the length), by the serrate sete of the swimming legs and the different shape and proportions of the fifth pair.

An average male measures .07 in, in length by .015 in. in depth. The cephalothorax is a little the widest before the middle, with angles rounded and terminating in a single acute spine. The second segment of the female abdomen is very short. The antennæ reach to the tip of the furca, and the antepenultimate segment of the right antenna bears a small hook at the tip in the male.

The outer ramus of the first pair of legs has three long bristles at its tip, of which the outer is dentate externally and plumose within, while the short spine at its base is dentate on both margins. The outer edge of this ramus is fringed with long delicate pubescence. On all the swimming feet the terminal seta is dentate externally. The characters of the fifth pair of legs of male and female are sufficiently shown by Figs. 17, 18 and 19 of the first plate.

This species is of especial interest and value, since I have collected it from pools in Southern Massachusetts, near Wood's Holl, and also from similar situations at Normal, Illinois.

The characters of specimens from these widely separate localities agree very closely, thus affording a most useful indication of the constancy of such characters as I have used in separating our species of this genus.

3. Diaptomus stagnalis, sp. nov. (Pl. vii Figs. 8, 10-12 and 14.) This species is the largest of its genus which I have seen, measuring .11 in. without the caudal setæ. It is apparently nearest to Diaptomus cæruleus (= castor) of Europe, but differs constantly from that form in several particulars in which the va rious figures of castor and westwoodii given by Baird, Liljeborg, Lubbock and Brady agree with each other.

The lateral angles of the cephalothorax are salient in the male and bifid in the female. The branches of the furca are nearly as broad as long, are hairy within and

1 Ueber zwei süss-wasser-Calaniden.

2

Microscopic Entomostraca. By C. G. Herrick. The Geological and Natural History Survey of Minnesota. The seventh annual report for the year 1878. Minneapolis, 1879.

about equal in length to the last abdominal segment. The antenna are robust and long, attaining the middle of the abdomen, and the antepenultimate joint of the male bears at its tip a stout conical process about one-third the length of the joint. On all the swimming feet the lateral spines of the outer ramus are bi-serrate, and the outer seta of the terminal three is strongly and sharply serrate without. This seta is about twice as broad as the others, and is but sparingly plumose within.

The inner ramus of the right leg of the fifth pair is rudimentary and unarmed, about half the length of the basal joint of the outer ramus.

The third joint of the outer ramus of the fifth pair of legs in the female is distinct and bears three set, while the inner margin of the tip of the preceding joint is coarsely toothed. The inner ramus is bi-articulate and terminates in two long feathered spines, which are longer than the whole ramus.

Several specimens were taken from pools in Central Illinois, in early Spring. All were red throughout.

4. Diaptomus sanguineus Forbes. (Pl. vIII Figs. 1-7 and 13.)

To the description of this species published in the first Bulletin of the Illinois Museum of Natural History, I will add but a few details. The posterior angles of the cephalothorax in the female are bifid, and its dorsal outline, regular in the male, is broken in the other sex by an elevation at the anterior margin of the penultimate segment, within which one of the levator muscles of the abdomen takes its rise. I know of no other Diaptomus possessing this character. None of the bristles of the anterior feet are serrate, although the lateral spines of the outer rami are so. The outer margins of these feet are not hairy.

Genus EPISCHURA, gen. nov.

(Pl. vi, Figs. 15, 16, 21–23, 25–27, and Pl. 1x, Fig. 8.)

In the general character of the legs, both natatory and clasping, this genus stands near Heterocope of Sars, but is remarkably distinguished from all the other Copepoda known to me by the development of the abdomen of the male as a prehensile organ. The abdomen has five segments, the second and third of which are produced on the right side as large and strong processes which act against each other like forceps, while a toothed plate on the fourth segment and a spatulate one on the fifth, assist to form a peculiar and powerful grasping apparatus. The cephalothorax has six segments, of which the last bears both the fourth and fifth pairs of legs. The head is very distinct from the following segment. The eye is single, small.

The female abdomen is four-jointed (the first joint very short), and is usually provided with a curved, cylindrical spermatophore, firmly cemented to the under side of the ovisac and extending upwards on the right, beside the third segment.

In the male the legs of the fifth pair are both one-branched, the left ramus threejointed and the right two-jointed. In the former the second and third joints oppose an enormous, curved and flattened process of the first. In the right leg the second joint is conical and hinged upon the first.

The fifth legs of the female are likewise one-branched and simple. They are three-jointed, small and unarmed, except at the tip where they are palmately toothed.

In all the remaining legs of both sexes, the inner ramus has but one joint, and the outer three.

The antennæ are 25-jointed and the right of the male is geniculate.

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