dorsal valve. The

b

D

have nearly equilateral shells. A depression or "sinus" frequently occurs down the centre of one valve, and a corresponding projection or "mesial fold" down the centre of the other. The sinus is almost invariably on the ventral, and the fold on the pointed upper extremity of the valve, is technically known as the "beak." In some forms the valves are close together; but in others, h a closed space (often striated across) occurs between the two. This is called the "area." See Fig. 88 and accompanying explanation. In the centre of the area, or under the beak of the ventral valve, there is frequently (as in the spirifers, &c.,) a triangular or circular orifice, the "foramen." This opening, in the species which possessed it, served for the passage of the pedicel by which the animal was attached to the sea-bottom. The foramen is situated, at other times, upon, or near to, the ventral beak, as in spirigera, &c. In many species again, it appears to have become closed by age; and in others, it is altogether absent. The line of junction between the upper part of the valves is termed the hinge-line. It is straight in some genera, (Orthis, Strophomena, Spirifer, for example,) and arched or curved in others, (Athyris, Rhynconella, Pentamerus, Terebratula, etc.) In many brachiopods, the shell is traversed by minute pores or tubular prolongations. When this is the case, the shell is said to be "punctate;" and when the pores are absent, it is termed "impunctate."

Fig. 88.*

The brachiopods possess, as their chief characteristic, a pair of long fleshy "arms," covered with delicate cilia, and either entirely confined in a coil within the shell, or capable of protrusion to a certain extent. In some genera, the inside of the dorsal valve carries peculiar spiral processes, or a shelly loop or other calcareous framework, for he support of these arms. A support of this kind is however wanting in many genera, or is otherwise merely rudimentary. The brachiopods differ essentially from the lammellibranchiate bivalves in the non-possession of distinct branchiæ or breathing gills. In existing seas the brachiopods are comparatively rare, the number of known species

• D=dorsal valve. V ventral valve. a, area; b, beak of ventral valve; ƒ, foramen; h-h, the hinge line; m, position of mesial fold; 8, position of mesial sinus.

not exceeding fifty; whilst the fossil species discovered up to the present time, amount to over thirteen or fourteen hundred. They constitute moreover, at least ninety per cent. of the bivalve shells met with in the lower fossiliferous rocks.

The following are the more important genera of Canadian occur. ence: Lingula, Orthis, Strophomena, Leptana, Spirifer, Athyris, Spirigera, Atrypa, Rhynconella, Pentamerus, and Stricklandia.

Lingula :--Shell: horny, thin, oblong, and nearly equivalve. Black and shining in our examples, and consisting largely (as first shewn by Prof. Sterry Hunt), of phosphate of lime. No internal calcareous appendages. This genus ranges from the Lower Silurian epoch into the present or existing period. Numerous species occur in our Silurian formations. L. quadrata, fig. 89, from the Trenton Limestone, Utica Slate, and Hudson River Group (Lower Silurian,) may be cited as a common example. Orthis:-Shell calcareous. Bi-convex or plano-convex; with straight hinge-line, and punctate surface No internal supports, properly so-called. This genus ranged throughout the Paleozoic age, but was most abundant during the Silurian and Devonian periods. The species have usually a more or less circular outline, with the surface of the shell marked by fine or course radiating lines.

Fig. 89.

Canadian examples are exceedingly numerous; more especially those belonging to 0. testudinaria, fig. 90, of the Trenton and higher divisions of the Lower Silurian series. Fig. 91 represents O. trice

A POPULAR EXPOSITION OF THE

naria; fig. 92, O. pectinella, and fig. 93, O. lynx, all of common occurrence in the Trenton Group. O. elegantula of the Niagara Group (Upper Silurian) is closely related to O. testudinaria, and has the general form of fig. 90. O. Vanuxemi, fig. 94, is a Devonian species. The Lower Silurian form, O. lynx, fig. 93, has the general aspect of a spirifer, but its mesial fold and sinus are marked by several plications, a character not exhibited by any of our Canadian Spirifers. It was formerly called Delthyris lynx.

Strophomena:-Shell, concavo-convex; hinge-line, straight; no internal supports. This genus ranges from the Silurian to the Carboniferous formation. Canadian examples are very abundant.

Fig. 95 represents S. alternata, a species of exceedingly common occurrence in the Trenton and Hudson River Groups (Lower Silurian.) S. filitexta is a closely related form. Fig. 96 exhibits another well-known species, S. rhomboidalis (=Strophomena and Leptana depressa,) from the Niagara Group and other Upper Silurian strata, and also from the Devonian rocks of Western Canada. In these latter rocks a few species of Chonetes and Productus, (genera allied to Strophomena,) also occur.

Leptana :-This genus (or rather sub-genus,) merely differs from Strophomena by the character and elongation of

its muscular impressions. L. sericea, Fig. 97, of the Trenton and Hudson River Groups, is a species of common occurrence.

Fig. 97.

Spirifer:-Shell with internal calcareous processes in the form of two spiral coils pointing outwards. Hinge-line straight, long; area well developed, with triangular foramen. The genus ranges from the Silurian to the Triassic (or Jurassic) epoch, but is chiefly characteristic of Upper Silurian, Devonian, and Carboniferous rocks. Fig. 98 represents S. Niagarensis of the Upper Silurian, and Fig. 99, S.

mucronatus of the Devonian series. Both are of common occurrence. The hinge-line of the latter is sometimes shorter (as compared with the height of the shell,) than is shewn in the figure. In our Western Devonian rocks, several other species occur: as S. duodenarius, with eight or nine rounded ribs on each side of the mesial fold; S. raricostatus, with two or three coarse plications on each side of the fold; S. gregaria, a small species, &c. These are figured and described by Mr. Billings in the Canadian Journal, vol. VI. Another common species of the Upper Silurian series, is S. radiatus. This differs chiefly from S. Niagarensis by its finer and more numerous plications. A third Niagara species S. sulcatus, has about eight plications on each side of the mesial fold, crossed by the rough and strongly-pronounced edges of the layers of growth.

Athyris:-The shell in this genus has internal spires as in Spirifer, but the hinge-line is curved, and the area is absent or rudimentary. Species range from the Silurian to the Triassic formations. Several occur in our Devonian rocks. One of the most common of these, A. clara, (Billings,) is represented in fig. 100. A. Maia is a somewhat similar species, but with a more developed or longer mesial fold and sinus, and with a slight space or false area between the beaks. These and other Devonian species are described in detail by Mr. Billings, in the Canadian Journal, Vol. V.

Fig. 100

Spirigera :-This genus or sub-genus differs from Athyris in having a perforation or foramen in the beak of the ventral valve. S. concentrica of the Devonian rocks is shewn in fig. 101. The genus Retzia is nearly allied to Spirigera, but the shells are smaller and strongly ribbed.

VOL. VII.

H

Fig. 101.

the

Atrypa-A good deal of uncertainty still prevails with regard to

limitation of this genus. proper Rhynconella, see below, but appears to possess internal calcareous spires, the points of which extend into the hollow of the smaller or dorsal valve. Fig. 102 represents an exceedingly common species, A. reticularis, of the Upper Silurian and Devonian strata, but chiefly characteristic of the latter.

Fig. 102,

Rhynconella-Shell, in general, strongly bi-convex. Hinge-line, curved; no area. No internal spires, but in the living species the arms are coiled spirally, the spires pointing downwards and inwards. The genus ranges from the Lower Silurian into the existing epoch. Fig. 103 represents a small form, R. plena, very common in the Chazy limestone of the Trenton Group, (Lower Silurian); and fig. 105, R. increbescens, a closely related species occurring abundantly throughout the Trenton limestone. In this latter species, the plications

Fig. 103.

Fig. 104.

Fig. 105.

on the shell are crossed by well-marked imbricating lines of growth. Numerous examples of this genus occur also in our Upper Silurian and Devonian strata. A modern species, found in the Post-Tertiary deposits of Eastern Canada, R. psittacea, is figured in the wood

cut 105.

Fig. 106.

Fig. 106 is a representation of the old Rhynconella hemiplicata of the Trenton Group, now referred by Mr. Billings to his new genus Camerella. It is characterized by a few broad plications on the lower part of the shell.