456 Mr. James Sowerby on a fossil She It 



Crenatula in many particulars, although apparently related to 

 them in its hinge : it consists entirely of a substance composed 

 of parallel perpendicular fibres, and much more conspicuously 

 so than Pinna or any other genus. 



From specimens in my possession I have made the outlines I 

 now present to the Society, Tab. XXV. It will be easily seen how 

 Cuvier and Brongniart had it not in their power to understand 

 the fragments they found, and how such fragments might mis- 

 lead any one. They are found in the flints and among the chalk, 

 both hard and soft, in all those places where chalk occurs in 

 England, &c. The shells are from an inch to two feet or more 

 in length, and generally very thin for their size, except at the 

 hinge and extremities. The hinge is generally lost or closely 

 enveloped in the chalk or flint, so as to be concealed, or else it 

 lies in confusion among the fragments. The part behind the 

 hinge or beak is of the finest possible thinness, while some of the 

 other extremities and the hinge are many lines thicker than the 

 more central parts. A shell of about three inches in diameter 

 may be about the tenth of an inch thick ; and I have one, which 

 if perfect would be about eighteen inches in diameter, not half 

 an inch thick : there are found fragments even larger, which 

 undoubtedly belong to this genus. I have one five-eighths of an 

 inch thick, from Wiltshire. Thus, if we were to infer from the 

 thickness of the fragments the size of the individuals to which 

 they must have appertained, supposing them to belong to the 

 genus Pinna, we must conclude that these testacea would have 

 been indeed ?nonstrous : but when it is known that they form a 

 peculiarly characterized genus of a less regular construction, 

 we may form a much more moderate idea of them. Upon a 

 fair calculation, the specimen measuring five-eighths of an inch 

 thick must have formed part of a shell two feet six inches long ; 

 whereas, had it been originally part of a pinna, if calculated 



according 



