THE DEVONIAN FOSSILS OF CANADA WEST. 143 
thing from the small circular aperture which occurs in the point of the 
beak of Spirigera. Oneach side is a short stout tooth, beneath which 
a strong nearly vertical septum extends a short way towards the front. 
These two septa are the dental-plates. Fig. 49 shews the form of the 
muscular impressions in A. Clara. At first sight they appear to be 
widely different from those of A. tumida, but this is owing to the 
greater thickness of the shell in the upper half of the ventral valve of 
this species. Since this species was described in this Journal, in May 
last, I have ascertained that the same variations in the form of the 
muscular impressions occur in the genus Spirifera. In the thick- 
shelled species it is deeply excavated, and is represented on the cast of 
the interior by an abrupt prominence, longitudinally or diagonally 
striated. 
In the thin-shelled species it is superficial, and presents a different 
appearance. There are other variations in the form of the scars in 
the ventral valve not represented in the above figures. Sometimes they 
extend nearly to the front of the shell, as is the case in an undescribed 
species from Anticosti, and in a Corniferous species of which I have 
some fragments. 
In the dorsal valve, fig. 50, there is a horizontal plate (the hinge- 
plate) just beneath the beak, with a triangular depression in the 
middle, from which a thin vertical septum extends about one-half the’ 
length of the shell. On each side of the central depression the hinge- 
plate of the specimen figured shows two short, slender, spine-like pro- 
jections, these are simply the bases of the spiral arms, which were 
here attached to the anterior edge of the plate. At the extremities 
of the hinge-plate are two small pits,—the sockets for the reception of 
the teeth of the opposite valve. The occlusor muscular impressions 
are four in number, and elongate oval, the anterior pair about the 
middle of the shell, and the posterior pair between the anterior and 
the beak. 
Fig. 48 is copied from Mr. Davidson’s paper in the ‘ Geologist,” 
Vol. I., Plate 12. Figs. 49 and 50 are from specimens in the collec- 
tion of the Geological Survey. 
In the sub-genus Merista the dental plates are connected by a 
peculiar arched plate, resembling a shoe-lifter, hence its name,—the 
shoe-lifter process or septum. (See fig. 53). In the species on which 
Prof. Hall founded his genus, Camarium, and also in some of the 
European forms, it extends from the beak downwards half the length 
