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THE DEVGNIAN FOSSILS OF CANADA WEST. 259 
Fig. 71. Fig. 72. Fig. 73. 
Fig. 71. Spirifera raricosta.—Conrad. Dorsal view. Fig. 72. Side view. 
Fig. 73. Ventral view of a specimen with the shell exfoliated. 
length of full grown individuals about one inch; width equal to or a 
little greater than the length. 
The dorsal valve is most convex in the middle and more or less 
flattened or concave towards the cardinal angles ; the area narrow 
sub-linear ; the beak small pointed and together with the area strongly 
incurved over the hinge line; the middle rib corresponding to the 
mesial fold of an ordinary Spirifera is usually very prominent, 
rounded or sometimes a little flattened on the top; its width at the 
front margin, in a specimen fourteen lines wide, is about five lines, and 
it is well defined and prominent all the way to the point of the beak ; 
the ribs next to it on each side, also reach the beak, but the two outer 
ribs become obsolete on approaching the hinge-line. 
The ventral valve is most gibbous in the upper half, the umbo 
rather small but prominent, and the cardinal angles not flattened. 
The area is somewhat variable in its dimensions; and cannot be seen 
when the shell has been compressed ; in large perfect specimens it is 
two lines high at the beak and half a line at the cardinal angles, and 
slopes outward at one angle of about 100° at its base, but is more or 
less arched towards the dorsal valve, so that its general direction is 
more nearly in the plane of the lateral margins. The beak is small 
pointed, always incurved over the area; the mesial furrows and four 
of the ribs extend quite to the point of the beak; the mesial furrow 
in all the specimens that I have seen is broadly rounded, while the 
lateral furrows are somewhat angular in the bottom. 
The surface is usually cvvered with small lamellose, somewhat 
rough ridges of growth; but in the more perfect specimens with fine 
imbricating concentric lines, of which there are from four to eight in 
one line; all of these are undulated upwards in crossing the ribs. 
