Pentamerus :-In this genus, the shell is prominently bi-convex, with arched hinge-line and large incurved beak. Internally it is divided by septa into several chambers. The genus ranges from the Silurian to the Carboniferous formations. P. oblongus, of the Niagara Group, is represented in fig. 107, the sketch 107 a shewing a ventral view of the internal cast. P. aratus, of the Devonian rocks, is figured in 108. This latter form is closely related to the well-known typical species P. galeatus.

Fig. 107.

Fig. 109.

Fig. 107 a.

Fig. 108.

Stricklandia :-This genus has been recently established by Mr. Billings. It includes certain more or less oval forms with nearly equal valves, formerly referred to Pentamerus. elongata, a Devonian species, is shewn in fig. 109. Other species occur in these and in the Upper Silurian rocks.

S.

Our Canadian formations do not appear, as yet, to have offered any examples of the wellknown genera, Crania, Calceola, and Terrebratula.

LAMELLIBRANCHIATA (or CONCHIFERA.)-Lamellibranchiate mollusks are marine or fresh-water animals of the acephalous type. They are provided in the adult condition with laminated gills or branchiæ for breathing purposes, (as seen, for example, in the socalled "beard" of the oyster,) and they secrete a bi-valve external shell. The two valves in most genera (those of the Ostreida and some Aviculidæ are the only exceptions) are of equal size, but always more or less inequilateral. (See under the Brachiopods above). These mollusks are exceedingly abundant in the fossil state, though less numerous than the brachiopods in the older rock for

mations. The known species obtained from the seas, lakes, and rivers, of existing nature, somewhat exceed three thousand, whilst nearly double that number of fossil species have been recognized. These latter, however, belong it must be remembered, not to one period, but to many successive epochs; although on the other hand, it is manifest that we see in them merely an incomplete record of the lamellibranchiate fauna of the Past.

In their classification, the lamellibranchiate mollusks fall into two leading sections and four groups, as follows:

(1) ASIPHONIDA

(1 a) Pleuroconcha.

(1 b) Orthoconcha.

(2) SIPHONIDA

(2 a) Integro-Pallialia.

(26) Sinu-Pallialia.

The animals of the first section are without the peculiar respiratory tubes possessed by the SIPHONIDA. These latter, for example, have a pair of short or long siphonal tubes, which assist in the process of respiration, and which admit in the Sinu-pallialia of extension beyond the shell.

The Pleuroconcha, (group 1), of which the oyster may be taken as a type, rest in their natural position with one valve below, and the other above, and thus approximate to the Brachiopods. They have in general but one large muscular impression in the centre of the inside of each valve. This forms a shallow pit, occupied by the muscle which keeps the valves closed. The common fossil known as Ambonychia radiata (fig. 110) may be cited, though doubtfully, for its true affinities are still obscure, as an example of this division. It is exceedingly abundant in the Hudson River Group of the Lower Silurian series.

Fig. 110.

The forms of the second group, or Orthoconcha, (as restricted above*,) are also without siphonal tubes, but their

The term Orthoconcha, it should be observed, is applied by some palæontologists to our groups, 1 b, 2 a, and 2 b, collectively-the forms of the two first of these being united under the subordinate group of Integro-Pallialia.

valves are right and left, instead of upper and under, as regards the normal position of the animal, and the muscular impressions are two in each valve. The fossil species known as Modiolopsis modiolaris fig. 111, so common in our Hudson River Group, belongs in all probability to this division. The genus Cyrtodonta of Billings, (with its sub-genus Vanuxemia), may also be referred to the Orthoconcha of this Section. Fig. 112 represents the Cyrtodonta Huronensis (var. subcarinata) of the lower part of the Trenton Group. Another and more remarkable species of this genus-widely known as the Megalomus Canadensis, of Hall occurs in great numbers in the Onondaga Salt Group, (Upper Silurian), of Canada West, and more especially in the neighbourhood of Galt. It is found chiefly in the form of internal casts, as shewn in the figures 113 and 113 a.

Fig. 111.

The lamellibranchs of the third group, Integro-Pallialia, have the upright (or right and left) position of the orthoconcha of Section I., but, unlike these latter, they possess a pair of short respiratory tubes. The muscular impressions, two in each valve, are connected, as in the forms of the

Fig. 112.

last group, by one uninterrupted shallow groove or "pallial impression,"-i.e., a continuous line without any bend or sinus in it. The existing fresh-water genus Cyclas, species of which occur in our

Post-Tertiary deposits, and especially in those of Western Canada, may be cited as an example of the present group. (See PART V.)

Finally, the mollusks of the fourth group, Sinu-Pallialia, possess a pair of long siphonal-tubes, capable of extension beyond the shell; and their two muscular impressions are united by a more or less deeply sinuated pallial line. Many of these lamellibranchs burrow

in the sand of the shores on which they live, between the tide-marks, with their respiratory tubes extending to the surface; and fossil examples occupying this upright position, and thus shewing the animals to have been fossilized in their original burrows, are met with in certain strata. As examples of the group, we may refer to Mya truncata, fig. 114, and to Saxicava rugosa, fig. 115, both of which are of exceedingly common occurrence in the Post-Tertiary deposits of Eastern Canada.

Pteropoda:-The living pteropods are swimming or floating mollusks, frequenting the open sea. Some few are naked, but the greater number secrete a delicate external shell (univalve,) and all possess a pair of fins or wing-like appendages for natatory purposes. In the pteropods with shells, the head is more or less indistinct. The Conularia is the only form of Canadian occurence, referrible, and that doubtfully, to this class. Fig. 116 represents O. Trentonensis of the Trenton Group. The shell in this genus is more or less conical and four-angled, furrowed longitudinally, and marked transversely by numerous straight or zig-zag lines. These latter often resemble rows of minute punctures. The genus extends from the Lower Silurian division into the Lias formation of the Mesozoic rocks.

Fig. 116.

HETEROPODA. The representatives of this class are regarded by many naturalists as forming simply an Order (Nucleobranchiata) of

the class GASTEROPODA. They constitute however a truly aberrant group, having affinities with the Pteropods on the one hand, and with both Gasteropods and Cephalopods on the other. Existing forms, like the pteropods, are of pelagic habit, swimming, by means of a fin-like appendage, in the open seas. The swimming organ is a modification of the gasteropod foot: see below. Some are without a shell, whilst others secrete one of a fragile and delicate texture,

Fig. 118.

Fig. 117.

sometimes provided, as in many gasteropods, with a lid or "operculum," by which the opening of the shell is closed when the animal withdraws itself within it. The fossil genera Maclurea, Bellerophon, and Cyrtolites, from certain characters which their shells appear to Fig. 119. possess in common with those of the modern genus Atalanta, are usually referred to this class; but much uncertainty still prevails with regard to the true affinities of these fossil types. The comparative solidity of the shell is opposed to their alliance with the Atalantide. Mr. Salter of the English Geological Survey, suggests, however, that Maclurea may have been a Heteropod with heavy shell, inhabiting the sea-bottom. Fig. 117, represents Maclurea Logani of the lower part of the Trenton Group; a is an inside view of the curious operculum often found detached. Fig. 118 is an example of Bellerophon expansus, and fig. 119 of Cyrtolites ornatus, of the Trenton and Hudson River Groups (Lower Silurian series.) By some palæontologists, the genera Bellerophon and Cyrtolites are considered identical.

GASTEROPODA.-The gasteropods have a distinct head; and all the typical species possess a fleshy expansion or foot on which they creep, and from which the class derives its name. The greater