Foerste.] 
312 
[May 1, 
is a frequent species in the Upper Silurian rocks of North America, 
while Ortliis Jiabellulum is unknown from the same horizons. 
We are not, however, equally certain that Ortliis jiabellulum is 
unknown from the Lower Silurian strata of this continent. Ortliis 
pectinella , Conrad, of New York, has muscular scars in the ventral 
valve, very well agreeing with similar scars described from typical 
European specimens. The surface of the ventral valve is also more 
or less flattened, but this flattening generally takes the form of a 
very broad and very shallow mesial depression, noticeable usually 
only along the anterior half of the shell and is not connected in 
any specimens examined with a concavity of the same region of 
the shell as in Ortliis Jiabellulum, Sowerby. 
Orthis biforata, Schlotheim. 
This species is of such great geographical and stratigraphical 
distribution that it is scarcely worth while to mention localities. 
It is abundant in the Clinton of New York. A single specimen is 
found in the Cumberland Gap, Tennessee, collections. Its cardi- 
nal angles were rounded ; seven radiating plications occupied each 
lateral lobe, two plications being on the strongly elevated mesial 
lobe, a faint third ridge lying in the space between. Presumably 
only one plication would be found in the sinus of this particular 
specimen, but this was not exposed. The specimen is 9 mm. long 
and 12.6 mm. wide. 
At Collinsville, Alabama, eight specimens were found, in all of 
which the hinge-line is narrower than the general width of the shell. 
The largest specimen here found is 18 mm. wide. Three dorsal 
valves have three plications on the mesial fold, four ventral valves 
had two. and one ventral valve, four plications in the sinus. 
In Ohio two forms occur, one,/. Daytonensis is the local repre- 
sentative of the Tennessee and Alabama specimens ; another/, re - 
versata is the local designation for a larger form, more like the 
larger Clinton forms from New York. In both of these forms the 
odd number of plications on the mesial fold and the even number 
of folds or plications in the sinus is the striking feature. 
At Hanover, Indiana, specimens of the same size as/. Dayton - 
ensis occur. The hinge line, however, usually exceeds the general 
width of the shell. Ten dorsal valves have three plications on the 
mesial fold ; eight ventral valves have two plications in the sinus, 
and one ventral valve has three plications. 
