262 PALEONTOLOGY OF NEW YORK. 



rostral region or pedicle-cavity, and produced into divergent ridges, usually two 

 on each side, and a fifth in the median axis. These may extend to the mar- 

 gins or disappear before reaching the middle of the valve and are variously 

 subdivided by vascular grooves and sinuses emanating from them. 



In the brachial valve the hinge-plate is small, similar to that of Rens- 

 SELiERiA and Amphigenia in general form, but is of relatively less size than in 

 the former genus and is not perforated by a visceral foramen opening beneath 

 the apex. Two veiy narrow, almost linear and closely submarginal dental 

 sockets extend nearly to the apex ; within them lie two broad, subtriangular 

 crural plates, which are divided by a triangular median fissure extending to 

 the bottom of the valve. The inner anterior angles of these plates bear the 

 slender crural processes, the extent of which is unknown. In mature individ- 

 uals the apical portion of the hinge-plate is peculiarly constructed ; the lateral 

 areas become more or less completely united, without altogether obliterating 

 the median triangular fissure, and above this point the surface is excavated 

 into a spoon-shaped cavity, when the development is extreme, or is trans- 

 versely angular in the average individual. At a short distance from the 

 hinge-plate and in the bottom of the valve there arises a low median ridge, 

 which continues for a short distance, separating the obovate, narrowly flabelli- 

 form scars of the anterior and posterior adductor muscles. The anterior scars 

 are considerably the larger, and their surface is longitudinally striated. The 

 vascular grooves and ridges are more obscurely developed than in the pedicle- 

 valve. 



Surface smooth or covered with fine concentric striae accompanied by stronger 

 wrinkles of growth. The inner lamims are sometimes marked by obscure 

 radiating striae near the margins of the valves. 



Shell-substance finely punctate. 



Type, Renssel(zria ? Johannis, Hall. Hamilton group. 



Observations. Several of the earlier species of Renssel^eria have an essen- 

 tially similar form to the representatives of this genus. This, however, is not 

 the broad-shouldered form of R. ovoides nor of Amphigenia, and none have 



