almost incomprehensible, some strata of certain rock formations seeming to be almost entirely composed of them. Amongst the recent species I know of no case analogous to this, except that of the dwellers in salt marshes and estuary mud; and I have no doubt that, were the mud-banks of our tidal rivers, and the swamps adjacent, suddenly petrified, we should, in many cases, find that the resulting stratified rock would exhibit as wonderful a concourse of once-living crustacean shells, as those which have just been spoken of. Mr. W. K. Parker has indeed conjectured, from a study of their fossil Rhizopoda, that the post-tertiary clays of the district round Peterborough constitute a littoral, brackish water-deposit; and it is remarkable that the Foraminifera there found, agree, to a very large extent, with those which I have myself taken in the salt marshes of our north-eastern coast. In strata much older than the post-tertiary, one would not, of course, expect to find species exactly identical with those now living. In washings of these clays, however, with which Mr. Parker has kindly supplied me, I have not been able to detect any shells of Ostracoda; it is, indeed, noticeable, that fossiliferous strata which are rich in Foraminifera are mostly poor in Ostracoda, and vice versa. Such, on consideration, one would naturally expect to be often the case. In all the salt marshes which I have had the opportunity of examining, living Ostracoda have been very abundant. If the net be passed carefully along the surface of the soft mud, so as to take up a thin layer, and the mass thus obtained be then washed through the net sufficiently to clear it of the fine ooze which will be found to constitute the greater part of its bulk, the residue will mostly consist of Ostracoda, living and dead, grains of sand, fragments of decaying vegetable matter, and very probably a good many living Foraminifera. This will mostly be the result wherever the water is brackish, and deposits a good deal of slimy mud, but in fresh water, or at the mouths of rivers, where the stream is clear and rapid, and does not produce much fine deposit, the Ostracoda, and other Microzoa, will be found much less plentifully. My belief is, therefore, that those strata which exhibit such very abundant and closelypacked remains of the smaller Cypride and Cytherida have most likely been formed in shallow, brackish lagoons, or at the mouths and deltas of rivers. The species of Ostracoda which I have found in these situations are, Cytheridea torosa (Jones), Cythere pellucida Baird, and Loxoconcha elliptica Brady; while in water, a little further from the saline influence, but still slightly partaking of it, it is not uncommon to meet with Cypris salina Brady, and Cypridopsis aculeata (Lilljeborg), as well as Entomostraca belonging to other orders. Some

crustacea of larger type are also commonly met with in brackish waters,* e.g., Corophium longicorne, Palamon varians, Mysis vulgaris, the common shrimp, etc.; but these, not having a hard, durable, calcareous investment, are not found in the fossil state. The Foraminifera which I have usually found in company with the Ostracoda above mentioned are, Polystomella striatopunctata, Quinqueloculina agglutinans, Trochammina inflata, Rotalia Beccarii, and Nonionina depressula; the relative abundance of these forms varying with the locality. The marshes of the Northumberland and Durham coasts are the only ones which have yet been carefully examined, and it is quite probable that other districts might yield different species.

By far the greater number of Ostracoda at present known have been described from fossil specimens, and the generic and specific characters have, of course, been taken almost exclusively from the external characters of the shell, the chief of these being, its general form and contour, mode of hingement, arrangement of lucid (or muscle) spots and style of surface. ornament. The general structure of the animals themselves has indeed been known sufficiently to form good grounds of separation between some well-defined families, such as Cypridæ, Cytherida, and Cypridinidæ, but the more minute anatomy indicative of generic and specific differences has, until recently, been very little understood or investigated. The family Cytherida, for instance, has been considered by most authors as consisting of some three or four genera (or sub-genera of Cythere), separated from cach other by shell-characters merely, and the family Cypride of two genera. But when we consider that, of the one hundred and thirty-seven species of Ostracoda now known as living in the waters of Great Britain, all except ten belong to these two families, and that amongst fossil species the disproportion is even greater, it is evident that, amongst so vast a number of species, many important differences of internal structure must exist, and that these stood in need only of careful investigation in order to form good grounds of generic subdivision. Accordingly, we find that much has of late years been done in this direction, more especially by Zenker and Fischer in Germany and Russia, and by Lilljeborg and G. O. Sars in Scandinavia. The researches

*See my paper on "Salt Marshes and their Inhabitants," in INTELLECTUAL OBSERVER, Vol. v. p. 26.

Any readers of the INTELLECTUAL OBSERVER living in the neighbourhood of salt marshes or muddy estuaries would materially contribute to the knowledge of a very important and interesting branch of natural history by collecting the Microzoa in the manner above described. The best material for collecting-nets is "crinoline;" the things captured may be preserved, either by drying, or by immersion in dilute spirit. I should myself be very glad of the opportunity of examining any such collections, and would with pleasure name the specimens obtained.

of the last-named author, embodied in his recent work on the marine Ostracoda of Norway,* are of particular interest to English naturalists, seeing that the marine faunas of the two countries exhibit a very close affinity. He has succeeded, after a most careful and painstaking investigation of the Norwegian species, in accurately ascertaining the minute structure of animals belonging to all the described fossil genera (excepting only, as he says, Cytheridea, which, however, I regard as belonging to the same genus as the forms he describes under the name Cyprideis), and has also established a large number of new species and genera. After a similar survey of the British marine and fresh-water species (in which I have received most valuable assistance from many collectors and naturalists whom I need not here stop to name), I have myself added many species to the list, and have found it needful also to propose some few new genera. A brief analysis of these I propose now to lay before the reader; but before doing so, it will be desirable to describe succinctly the general type of structure of the Ostracoda.

Each member of the class Crustacea is considered, typically, to be divisible into twenty-one annular segments, or somites, seven of which belong to the head (cephalon), seven to the thorax (pereion), and seven to the abdomen (pleon). But it is only in very few cases, and these amongst the more highly organized members of the class, that these segments, or their rudiments, are discernible. In most cases some of the segments are fused together, so that their real nature is to be recognized only by the presence of certain limbs or appendages which indicate their existence. Thus, under the hard, calcareous carapace, or shield, which protects the head and back of the lobster, we find gathered all the cephalic and thoracic members of the animal, and so we learn that the great dorsal buckler consists, in fact, of all the cephalic and thoracic segments of the body, cemented into one strong protecting plate. This principle, infinitely modified, runs through the whole class; but we also find that, in addition to the coalescence of various segments, other segments are often entirely absent, their presence not even indicated by the existence of any limbs or appendages. So that it is only by the careful study and comparison of the whole group that the real nature and homologies of any particular organ can be made out. The appendages of the twenty-one segments of the typical Crustacea may be tabulated as follows:-1, Eyes; 2, 3, First and second antennæ; 4, Mandibles; 5, 6, Two pairs of jaws; 7, 8, 9, Foot-jaws; 10-14, Ambulatory legs; 15-20, False or abdominal feet; 21, Tail-piece or post-abdomen. In the "Oversigt af Norges Marine Ostracoder" af G. O. Sars, 1865. VOL. XII.-NO. II.

I

Ostracoda the segments of the body are almost entirely obsolete, the body itself being soft, and entirely covered by two usually hard and calcareous, but sometimes thin and horny, valves.

The family Cypride, which includes almost all the freshwater, and a few of the marine Ostracoda, may be taken as the basis of our description. The various organs will be better understood by reference to the plates, where they are figured in detail.

The first limb, or upper antenna (Fig. 1, b), here consists of a tapering seven-jointed curved appendage, which bears at its apex a dense bundle of long plumose setæ. The second, or lower antenna (Fig. 1, c), is usually stouter, four-jointed, strongly bent or "geniculate," clawed at the extremity, and adapted for walking, though in many cases it is also provided with a lash of setæ, which adapts it likewise for swimming. The mandible (Fig. 1, d) consists of an elongated triangular body, the base of which is directed downwards, and is divided into numerous teeth. From one side springs a four-jointed palp (Fig. 1, f), whose basal joint bears one of the branchial organs (Fig. 1, e). Of the two pairs of jaws, the first (g) is the larger, and is divided into four segments; to it is attached a large branchial plate (h), the principal breathing organ of the animal. The second pair () is small, and has a non-articulate palp, which, in the male, is often modified into a prehensile organ. In some genera this pair of jaws bears also a small branchial plate. There are two pairs of feet, the first (j) clawed at the extremity, and adapted for walking; the last () slender, flexuous, and always tucked up within the shell. The post-abdomen (m) consists of two flattened elongated rami, which are very movable, strongly clawed at the extremity, and lie side by side, mostly within the shell. There are sometimes two eyes (a), but these are mostly confluent. The ovaries (o) lie round the body of the animal, directly beneath the shell. The copulative organs of the male are of very curious and complex structure, and have mostly attached to them a mucous gland (Fig. 4), consisting of a double central cylinder, and several whorls of radiating filaments. The shell is thin and fragile, and mostly somewhat reniform or ovate in shape, devoid of sculpture, except sometimes a fine impressed punctation; occasionally densely hispid or even spinous.

We shall now briefly point out the characters in which the other families chiefly differ from the Cyprida.

CYTHERIDE. This family includes by far the greater number of the marine Ostracoda, and some few species are inhabitants of fresh water. Both pairs of antennæ (Fig. 2, b, c) are en

tirely destitute of the filamentous brushes which give swimming power to many of the members of the preceding family; but the upper antennæ (b) are beset with strong marginal spines or setæ ; and the lower (c) are provided with a single long tubular flagellum, which communicates with a poison-gland (v), situated in the anterior portion of the body of the animal. There is only one pair of jaws (g), similar to the first pair of the Cypride. Three pairs of feet (r, s, t), all of which protrude from the shell, are very similar in form, though increasing in length from the first to the last, and are adapted for walking. The postabdomen is composed of two very small and inconspicuous lobes (m). The ovaries are not produced between the two valves. The male copulative organs are exceedingly large, and of complex structure, and are not provided with a mucous gland." The shell is mostly more or less quadrangular in outline, but sometimes ovate or subtriangular; very variable both in structure and in external ornamentation.

CYPRIDINIDE.-The antennæ are here exceedingly large and muscular, the upper pair (Fig. 10, b) often bearing very long and slender terminal setæ, as well as a densely-tufted auditory seta. The lower (c) is especially powerful, composed of an excessively large and muscular basal joint, to which is attached a nine-jointed branch, bearing numerous plumose setæ, which constitute it a powerful swimming apparatus. The mandible proper is rudimentary, but its palp (f) is developed into a large prehensile limb. Three pairs of maxillæ, of very variable structure (g, h, i). One pair of feet, of very peculiar structure (k), forming a long, flexuous, annulated, vermiform process, bearing several toothed spines at its extremity and evidently homologous with the second pair of feet of the Cypride. The post-abdomen is composed of two large closelyappressed plates (m), which are powerfully clawed along the posterior margin. Eyes two (a), pedunculated. Shell subovate or subspherical, having a distinct beak, with a large under. lying notch in front, through which the antennæ are protruded whilst swimming.

Some of the members of this family have very slight swimming power, and live chiefly amongst mud; others are very agile swimmers, and are often taken in the towing-netmore especially at night-near the surface of the sea. They seem, indeed, to contribute very materially to the production of the wonderful phosphorescence of the tropical seas.

CONCHOECIADE.-Closely allied to the preceding family. The upper antennæ are, however, in the female, very small; the lower antennæ almost exactly as in Cypridina. Mandibles distinct, narrow, and toothed; palp large and pediform, terminating in long claws. Two pairs of jaws. Two pairs of feet,