Foerste.] 
298 
TMay 1, 
they meet the cardinal line. These corrugations become longer and 
more distinct towards the cardinal extremities. These forms are 
now placed under Leptcena ti ansver salts asvar. prolongata, although 
more distinct than most of the other varieties of this variable species. 
In our conception, therefore, Leptcena sericea is a Lower Silurian 
species of wide distribution in North America, and presenting 
moderate degrees of variation, distinguished chiefly by the moderate 
convexity of the ventral valve and the convex lateral outlines of the 
muscular scars of the dorsal valve. Leptcena tr ansver salts, occurring 
in Upper Silurian rocks of North America, presents a very great 
range of variation, and is distinguished in part by the greater de- 
gree of convexity of the ventral valve but chiefly by the parallel 
sublinear lateral outlines of the muscular scars of the dorsal valve. 
The internal characters, however, of so many of our American 
forms are unknown, that their relationships are confessedly impos- 
sible definitely to determine. The variety prolongata has its coun- 
terpart in the variety Duvalii , Davidson, of Great Britain. 
Strophomena rhomboidalis, Wilckens. 
This species was described by Hall from the Clinton of New York 
under Leptcena depressa , there considered a distinct species. 
Leptcena quadrilatera , Shaler, of Anticosti, is the same form, also 
of small size. The corrugations of the surface are well marked and 
the geniculation near the anterior border is sharply defined and 
abrupt. 
Strophomena rhomboidalis is represented in the collection from 
Cumberland Gap, Tennessee, by one characteristic specimen, 5 mm. 
long and 8 mm. broad. The surface — not considering the six con- 
centric folds — of the valve is quite flat until the last fold at the an- 
terior geniculation is reached, which fold is so high as to give in 
general a somewhat concave aspect to that part of the shell which 
lies posterior to the geniculating border. From the geniculation 
the shell descends almost perpendicularly for 1.4 mm. to the an- 
terior margin. A number of shells occur which suggest the young of 
this species. One small specimen is 8.5 mm. long and 10.8 mm. 
wide, has four radiating striae to each 2 mm. in width, small faint 
striae alternating with these. In some of the larger specimens 
no surface features at all are preserved. These valves seem to pre- 
serve all their youthful characteristics, except size. All the valves 
found are ventral and convex. The convexity over the greater 
