218 ANNAL8 NEW YORK ACADEMY OF SCIENCES 



general aspect of this species is that of Producti of the semireticulatus 

 group, only somewhat abnormal in that the typical semireticulati are 

 broad, subquadrate shells and marked by numerous regular concentric 

 wrinkles passing across the visceral area from one large ear to the other. 

 P. elegans, however, has a narrow, more gradually expanding shape; the 

 ears are small and the concentric wrinkles few and irregular. The costjE 

 are strong and subequal, but tend to be discontinuous over the visceral 

 parts and to have tlie appearance of appressed spines. While not one of 

 the typical semireticulati in expression, P. elegans is at least typical 

 Productus. The diagnostic character is internal and consists of a par- 

 tition passing completely across the interior of the shell. This structure 

 appears to be an outgrowth of the dorsal valve from the geniculation, 

 where the flattened visceral area abruptly joins the lateral areas. It lies 

 in the same plane with the visceral area and appears, as it were, an 

 extension of it. 



This structure frequently forms a plane of dehiscence when specimens 

 are broken out of the rock, the visceral area of the dorsal valve, the vis- 

 ceral area of the ventral valve and the diaphragm remaining on one 

 piece, while the lateral and anterior extensions of both valves (which 

 are almost in contact), together with the mold of the diaphragm and of 

 the visceral area of the dorsal valve, remain on the other. The dia- 

 phragm and visceral area of the dorsal valve, while essentially on the 

 same plane, are readily distinguishable, being separated by a slight ridge 

 (or groove) and marked by different sculpture, the regular strong costse 

 of the external shell being replaced on the diajDhragm by fine radiating 

 striae. 



There is no doubt that this type should be distinguished from true 

 Productus, but there may be some question as to whether it is not already 

 covered by Waagen's genus Marginifera. "Waagen's description reads as 

 if Diapliragmus might be an extreme example of Marginifera, but there 

 ■can hardly be a doubt, I believe, that Diapliragmus is something distinct 

 from Marginifera splendens, the typical species of Marginifera. Indeed, 

 I am inclined to suspect that Waagen, who apparently did not have 

 access to specimens of the American species and was working from the 

 literature alone, may have been led to misinterpret the figures and de- 

 scriptions of N. splendens so as to imagine that the bevel of the dorsal 

 valve was an internal feature exposed by fracture rather than an external 

 feature which is shown on the outside of all perfect, well-preserved speci- 

 mens. However this may be, it seems to me that Marginifera must ad- 

 here to the characters shown by M. splendens. and that that species is 

 clearly a distinct type of structure from Diapliragmus. 



Type. — Diapliragmus elegans. 



