KEPTILIAN AGE. JTRASSIC PERIOD. 



75 



beak ; area triangular, arcuate, extendiug close up under the curve of the beak, and provided with a distinct 

 cartilage depression ; muscular scar small, shallow, oval, and located near the anterior side ; surface ornamented 

 on the gibbous back of the umbo by distinct, irregular, radiating stri», usually extending to near the middle on 

 mature specimens, while the space between this and the ventral margin has only moderately distinct concentric 

 strife and marks of growth. Upper valve ovate, nearly flat on the outside, or a little convex near the beaks, and 

 more or less concave near the middle, usually concave on the inner side towards the cardinal extremity, which is 

 thick and truncated ; surface with rather distinct concentric marks of growth. 



Length, from the most prominent part of the umbo to the ventral extremity, 2.70 inches ; breadth near the 

 ventral extremity, about 1.20 inch ; convexity, 0.73 inch. (Type No. 1881.) 



C 



D 



Gryphsea calceola, var. nelrascensis. 



Side view lower valve. B. Under side same. C. Inside same. D. Inside view of an upper valve. 

 E, Upper view of a small specimen with the two valves united. 



The normal form of this shell, as may be seen by the above cuts, agrees so very 

 nearly with Quenstedt's G. calceola, that we cannot but regard it as most probably 

 only a variety of that species. It has the same narrow, elongated, arcuate form, 

 radiately striated umbo, and general appearance of the fully developed specimen 

 of G. calceola, represented by Quenstedt's fig. 1, pi. 48, above cited, excepting that 

 the beak of the under valve seems never to be quite so arched over and produced ; 

 while the sulcus along its anterior side of the exterior is not continued so nearly 

 to the point of the beak. 



Along with the form above described, we have from the same and other localities 

 a few specimens with the point of the beak slightly truncated by a small scar of 

 attachment, much as we see in fig. 4, pi. 48, of Quenstedt, referred by him pro- 

 visionally to Ostrea calceola, Goldf. 



At some localities nearly all the specimens have the beak truncated, and many 

 of them seem to have been attached by so large a surface as to have entirely 

 obliterated the umbo of the under valve, thus giving them all the characters of a 

 true Oyster. Fig. la, h andd, PL III, represent some of these specimens, one of which, 

 fig. Id, will be seen to present very nearly the form and general appearance of fig. 

 2, pi. 48, Quenstedt, the most extremely abnormal type of the series. Between 

 these extremes we find every intermediate gradation, so that it seems to be im- 

 possible to base specific distinctions upon these differences. We are, therefore, 

 inclined to regard these shells as all belonging to one variable species, the diff"er- 

 ences being probably caused by the more or less favorable conditions of different 



