ATHYRIS OF THE HAMILTON GROUP. 
291 
Atliyris cora. 
PLATE XLYII. 
Atliyris cora : Hall, Thirteenth Report on the State Cabinet, p. 94. 1860. 
Shell depressed suborbicular or transversely broad-elliptical, moderately 
gibbous; valves subequally convex. 
Ventral valve somewhat more elevated than the dorsal, gibbous upon 
the umbo and much elevated above the opposite valve ; apex incurved ; 
outline regularly curving to the cardinal and lateral margins ; mesial 
sinus more or less strongly defined, sometimes shallow and sometimes 
subangularly depressed and reaching nearly or quite to the beak. 
Dorsal valve a little less elevated than the opposite, regularly convex 
and curving to the margins : the mesial fold is moderately elevated, 
sometimes rather abruptly defined at its margins, and flattened or a 
little concave above. Surface marked by fine concentric lines of growth 
and a few stronger parallel ridges or undulations. 
The specimens are in the condition of partial casts, or with the shell 
partially exfoliated, so that the original exterior is not fully known. In 
these specimens, when the shell is exfoliated, there is a depressed line 
marking the centre of the mesial fold, and in some specimens this fea¬ 
ture occurs without any appreciable elevation. So far as can be observed, 
the muscular impressions in the ventral valve are proportionally more 
expanded than in the A. spiriferoides, and those of the dorsal valve wider. 
The vascular impressions are sometimes very finely preserved beneath 
the inner laminae of the shell, and the surface of the cast is strongly 
papillose. 
In a very symmetrical specimen, the proportions of length and width 
of the dorsal valve are as eleven to fifteen, and the length of the ventral 
valve of the same individual is a little more than twelve. The greatest 
width of specimens is about -ff of an inch. 
This species differs from the A. spiriferoides in the character of the mesial fold 
and sinus, in the proportional greater convexity of the ventral valve, and greater 
elevation of the umbo ; and there are probably other differences which would be 
discovered in the perfect exterior, or in the entire muscular impressions. 
