192 REPORT ON THE STATE CABINET. 



dorsal valve, and the prismatic or punctate texture of the shell ; having 

 at that time an imperfect knowledge of the interior structure of the 

 ventral valve. Now this species has no area on the ventral valve ; but 

 it has a V-shaped pit supported by a septum precisely as in Pentamerus ; 

 while both the muscular impression of the dorsal valve, and the structure 

 of the hinge-plate, are precisely the same as in Renssel^ria. I cannot 

 see, therefore, upon what gi'ound this shell should be associated with P. 

 Hratus, P. c/aspensis, P. canadensis and P. anticostensis , which have a 

 straight hinge-line, an area on the ventral valve with a septum supporting 

 a short V-shaped pit, a different muscular imprint and hinge-processes of 

 the dorsal valve, and a fibrous or lamellose structure of shell, and are 

 otherwise externally marked in a very characteristic manner. 



I have taken some pains to bring together specimens of P. (R.) elon- 

 gatus, and we have now the means of knowing its real characters to a 

 very considerable extent. As before observed, the dorsal valve presents 

 all the characteristics of Renssel^ria in its hinge-structure and muscular 

 impressions : there is the same narrow foramen beneath the hinge-plate, 

 a pedicle-opening, and the filling of this is preserved in several specimens 

 of the casts ; while this portion of the separated valve shows the same 

 features in a most unequivocal manner, and it is quite impossible to point 

 out any characters by which it may be separated from the same part of 

 the shell in E-ENSSELiERiA. The crura are free from near their origin, 

 and have been traced for some distance within the cavity, and gently 

 curving to the ventral side ; but their termination is unknown. When, 

 however, we examine the hinge of the ventral valve, we find a modifi- 

 cation of the dental plates, which are prominent in Renssel^ria (as 

 shown in fig. 3, g, h and m, Plate cvin, Palseontology of New York, 

 Vol. iii), but which are here produced anteriorly and united at their 

 dorsal margins. The incij)ient septum in the bottom of the valve of 

 Renssel^ria becomes developed, and sustains the united bases of the 

 dental plates ; and we have the V-shaped pit and central septum of 

 Pentamerus. 



In the separated valves of this species, the margins of the triangular 

 fissure of the ventral valve are as clearly defined, and as free from area, 

 as are the valves of Athyris or Meristella. 



Generic significations must have some limit, and we cannot consent to 

 miite three so widely differing and well marked types as P. Hratus, P. 

 gaspensis, etc. with P. ohlongiis and P. lens on the one hand, and P. elon- 

 gatiis, on the other. 



