so PALEONTOLOGY OF NEW-YORK. 



impressions, as S. perplana, S. textilis, S. alveata, and in the Lower Helder- 

 berg group the S. beckii. 



•There are still some other forms which make a distinct natural group, 

 in being all resupinate, and in having the dental lamellae continued in 

 an elevated border which nearly surrounds the divaricator muscular 

 impressions. We have but two representatives of this group in the higher 

 formations, the S. ampla and S. viduus, the former of which is perhaps 

 identical with S. headleyana and S. pundulifera of the Jjower Helderberg 

 group. The S. leavenworthana, which is a very marked species, belongs to 

 the same group ; and by a little extension of the characters, the S. striata 

 of the Niagara group may be included : the two latter have a cardinal 

 process more like SxREPTORirYNCHUs. By a farther step, we may include 

 the species I have described as Strop /lodmta semifasciata, which has an area 

 on both valves, and on each one a triangular foramen closed by a convex 

 deltidium. With all these features of Streptorhynchds, together Avith a 

 cardinal process similar to other species of that genus, I have found 

 that the hinge-line is crenulated for one-third the length on each side of 

 the centre. The principal ground on which this one could be separated 

 from Streptorhynchus is the crenulated hinge-line ; although the striao 

 are somewhat dissimilar, and we do not fully know the ventral muscular 

 impressions. Were we to admit that Streptorhynchus may have a crenu- 

 lated hinge-line, then going one step further, and taking the form of 

 muscular impression and cardinal process, leaving out of consideration the 

 closed area, we may include in the Genus Streptorhynchus other species 

 enumerated in this group. This would leave for Strophodonta a more na- 

 tural assemblage of species than are at present united under that genus. 



In every one of the characters used to distinguish Strophomena, Stro- 

 phodonta, Lept^na, and Streptorhynchus, there are gradations or varia- 

 tions on the one side or the other, which ally the species so far that it 

 becomes often no easy task to indicate the limits of the genera ; and the 

 additional information obtained from the internal structure of numerous 

 apecies has not served to clear up the difficulties. 



