No. 186.] 



13 



Surface marked by concentric laminae of growth, and often by fine 

 radiating striae. 



MeQAMBONIA CARDIIF0RMI3. 



The following species from the Lower Helderberg group have been 

 referred to this genus, some of them having been determined from 

 external features alone, and others from the internal casts and a partial 

 exposure of the hinge-line. See Plates 49, 49 a, and 50, Palaeontology 

 of New- York, Vol. iii. 



Megambonia suborbicularis. 

 M. spinneri. 



M. aviculoides. 



M. rhomboidea. 



M. mytiloidea. 



M. ovoidea. 



Megambonia obscura. 

 M. lata. 



M. oblonga, 



M. cordiformis. 



JM. ovata. 



NOTE. 



In this connection, I may also notice the Genus Ctpricardites of Conrad, 

 published in 1841 (Annual Report on the Palaeontology of New-York, p. 51), 

 which has been almost entirely overlooked. The description is as follows: 



" Genus Ctpricardites. Equivalved, profoundly inequilateral : hinge with 

 *' four or five unequal cardinal teeth, anterior one largest and most promi- 

 " nent ; lateral teeth short, and very remote from the cardinal teeth." 



" This genus is allied to Pterinea of Goldfuss, but is never properly 

 *' alated, nor has it the very large muscular impressions of that genus : the 

 " cardinal and lateral teeth are also diifcrent ; the anteri.r cicatrix is often 

 " deeply impressed ; the posterior one not visible in casts of the interior. 



