LAMELLIBEANCHIATA OF THE LOWER MAKLS. 117 



all the specimens which I have observed from within the limits of" the State 

 have been more or less decayed. In fact, they are usually but little more 

 than casts; but all preserve a very decided and well marked feature of the 

 exterior shell, presenting strong, distant, and rather evenly distributed 

 concentric ridges, which vary on different individuals from a sixteenth to a 

 twelfth of an inch in width. The hinge-plate, as indicated on specimens 

 where a single valve only is preserved, seems to have been tolerably wide, 

 but the hinge features are never sufficiently preserved to give their char- 

 acters. The muscular imprints are large and but faintly marked. 



The second condition under which we see this form is as internal casts. 

 In these the form is much the same as in the other, with, perhaps, a greater 

 degree of variation, the beaks being sometimes nearly subcentral. The 

 shell seems to have been very thick, the muscular scars very deep, and the 

 pallial line remarkably thickened, the beaks prominent, with an extended 

 rostral cavity beneath the surface, never showing any signs of the strong 

 concentric undulations. 



It is difficult to reconcile these two fonns, which often occur together 

 at the same locality and present such different aspects, with one's ideas of 

 specific variation; but there have doubtless some conditions existed in the 

 circumstances of their preservation which has caused the shell to be 

 entirely destroyed in the one case while it has been partially preserved in 

 the other, and which have at the same time prevented the surface markings 

 from affecting the internal cast. Mr. Conrad's species, C. lintea, described 

 from a small specimen from Alabama, appears to be the same species as 

 the New Jersey shell, and neither of them would appear to differ materially 

 from the Prairie Bluff specimens visually found in collections, and undoubt- 

 edly representing the same as that used by Dr. Morton. Mr. Gabb 

 considers Conrad's C. Bepleyana as a synonym of this species also, (see P. 

 A. N. Sci., Phil., 1876, p. 310), and expresses a doubt if C. Carolinensis is 

 not also the same. 



Formation and locality. — The form referred to, C. vadosa, occurs in the 

 Lower Green Marls at Holmdel, Freehold, and Burlington, New Jersey. 

 The C. Mortoni has been noticed in the same beds at Holmdel and Free- 

 hold, but may occur at many other places. 



