ATHYRIS OF THE HAMILTON GROUP. 391 



Atliyris cora. 



PLATE XLVIL 

 MhyrU cora : Hali, 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. 

 DoRS.\L 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 margin^, 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 jf of an inch. 



Thi.s species differs from tlic A. spiriferoides in the character of the mesial fold 

 and simis, in the proportional greater convexity of the ventral valve, and greater 

 elevation of the umbo ; and there arc probably other differences which would bo 

 discovered iu the perfect exterior, or in the entire muscular impressions. 



