46 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Genera related to Hippurites. 



In searching out the affinities of a problematic fossil shell, it is 

 desirable to inquire, first, whether any similar, but less abnormal, 

 forms occur in the same stratum with it, or in formations imme- 

 diately older or newer. For it may be doubted whether any quite 

 isolated types exist in nature ; and, although no well-disciplined 

 naturalist dreams of the transmutation of organic forms, yet by those 

 who regard genera as "ideas of the creating mind," there is held to 

 be a relation of interdependence between those "ideas," as regards 

 their development both in time and space, giving rise to a succession 

 of forms, which may easily mislead a superficial observer to suppose 

 they are related in the way of ancestry or descent. 



Therefore, although we disbelieve the doctrine of transmutation, 

 our first inquiry is — Are there any fossil shells which look like the 

 progenitors or descendants of the Hippurite ? We think it may be 

 shown, that, by a complete series of cognate forms, the Cretaceous 

 Hippurites are connected with the Oolitic Dicerata and the Tertiary 

 ChamcB. 



These forms belong to at least five genera ; and some of the species 

 are more problematic and extraordinary than the Hippurite itself. 



Radiolites. — The genus most nearly related is the Radiolites of 

 Lamarck {SphcBvulites of De la Metherie) . 



Figs. 10 & 11. — Radiolites mammillaris, Matheron. Reduced \. 

 From the Lower Chalk, S. Mamest, Bordogne. 



Fig. 10. 



Fig. 11. 



Fig. 10. Interior of lower valve. Fig. 11. Interior of upper valve. (See also 

 figs. 13 & 14.) — I, ligamental inflection ; m, pallial line ; c, c, cartilage- 

 pits ; a, a', adductor impressions and processes ; t, t, teeth and dental 

 sockets ; u, umbonal cavity. 



In general form the Radiolites resemble the Hippurites, but are 

 more squamose or foliaceous externally ; and the upper valve is not 

 porous, differing but little in structure from the lower valve*. The 



* Mr. D. Sharpe has called ray attention to the existence of a third, superficial, 

 shell-layer in all the genera of Rudisfa, similar to the " sub-epidermal " layer, 

 described by Dr. Carpenter, in Chama and other bivalves. In the Radiolite (PI. IV. 



