BEEDE: VARIATIONS OF SPIRIFER CAMERATUS. I05 
posits of shell or thick plates, and the cavity formed by the jut- 
ting of the cardinal area is one general posterior cavity, extending 
across the shell, only slight ridges being 
present behind and below the teeth. This 
form is illustrated in figure 2. 
In other specimens the posterior por- 
tion of the shell is much thickened; mus- 
Fig. 2. cular depression not extending beneath the 
beak or hinge line, that portion of the cavity being filled with shell, 
which _ encroached on the muscular area, until it is small and 
nearly circular; the small mesial ridge is 
much less distinct than in the previous spec- 
imen. This thick deposit of shell also 
forms a heavy support for the teeth, and 
divides the posterior cavity into a right and 
Fig. 3. left cavity. See figure 3. 
A minute, young specimen figured in the plate (figure 31) 1 
nearly destitute of markings. The mesial fold and sinus are pres- 
ent, but have no indication of striz upon them; there are indica- 
tions of the three plications on each side of the fold; no other 
markings visible. Cardinal area visible, but ill-defined, and the 
entire shell has a glossy indefiniteness about it.. The following 
measurement of specimens will illustrate the great variation in 
form of this species: 
Length. Breadth. Convexity. 
30 mm. 60 mm. 20 mm. 
50 31 20 
59 35 28 
52 Sih 23 
14 18 8 
PAVORA CE LOL 31,00. 9 32 42 21 
MenyeVOUNE: <ilu. 4 3 2% 
One would naturally look for the degree of deposition of shell 
material in the ventral valves of the above specimens to be propor- 
tionate to the age of the shell, but from the material at hand I am 
able to draw no such inferenca. 
