282 



PALjEONTOLOGY of new YORK. 



with the muscular area not strongly limited ; consisting of two broad flaljel- 

 late diductor scars enclosing an elongate, more distinctly defined adductor. 

 The faintness of the limitation of this area is in marked contrast to the sharply 

 defined muscular area in the corresponding valve of Lept^na. In the brachial 

 valve the cardinal process is more closely sessile than in Lept^na, and there is 

 frequently a linear callosity between the branches. The posterior adductor 

 scars have the arborescent markings of Leptena rhomboidalis, and these impres- 

 sions are the only ones well defined, the anterior scars being narrow and rarely 

 retained with distinctness. From the anterior margin of the muscular area 

 radiates a series of irregular furrows and nodose ridges, which are to some ex- 

 tent of vascular origin. 



Type, Leptena alternata, Conrad. Trenton and Hudson River groups. 



Fig. 19. Jiajinesquina Julcesi, showing the brachial ridges. 

 After Davidson. 



Observations. There are some shells, a small number of species however, 

 which combine to some extent the characters of both Lept^na and Rafines- 

 QUINA. We may instance Leptana deltoidea, Conrad, and Strophomena unicostata, 

 Meek and Worthen, in which there are not only low, concentric corrugations 

 on the exterior, but in the latter species the interior of the brachial valve 

 has more distinctly the impress of Lept^ena than of Rafinesquina. There 

 are concentrically wrinkled species in the genera Strophomena, Stropheodonta 

 and Strophonella, but that character will jDrove of little value except for a 

 subsidiary grouping of species. The extravagant development of this feature 



