m 
PALAEONTOLOGY 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. punctulifera of the Lower 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 Streptorhynchus. By a farther step, we may include 
the species I have described as Strophodonta 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 Streptorhynchus, together with 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 striae 
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 
species has not served to clear up the difficulties. 
