No. 200.] 
109 
Leptcma^ has one valve convex, the other concave; a narrow cardinal 
area and a small triangular perforation. The form is generally the seg- 
ment of a circle. 
Delihyrisj has both valves convex; a cardinal area and triangular per- 
foration. The shell is often much produced, forming a long straight 
hinge line, and the shape is generally triangular. 
OrihiSj has no cardinal area; the valves slope from the apex, and there 
is a narrow perforation under the beak. 
Jitrypaj has the valves sloping from the apex, no cardinal area, and 
the beak of one valve is bent over the apex of the other; there is no 
perforation. 
The genus Producta contains very few species in the lower part of the 
silurean system, and they have little diversity of form. The amount of 
species gradually increases in the upper strata, and finally they exhibit their 
maximum in number of species and individuals in the carboniferous sys- 
tem. The four Brachiopodous genera I have described, soon disap- 
peared after the Carboniferous epoch, and the true Terbratula were then, 
for the first time, called into existence in great profusion. 
The course of our investigations having resulted in the conviction, 
that the rocks of New-York, with the exception of the upper part of 
the Catskill mountains, terminate with the upper Ludlow rocks of Mur- 
chison; we have an additional argument in favor of the universal nature 
of ancient formations, and the contemporaneous deposition of our coal 
strata with those of Europe. This correspondence Professor Eaton has 
inferred from the identity of the fossil flora, but the proof does not end 
here. While the same kind of plants flourished upon the islands where 
now are the continents of Europe and America, the same species of 
shells existed in the waters which girded them. Thus in the ironstone 
layers beneath the coal of Tioga county, the common European bivalve 
Producta scahricula abounds just as it does at Coalbrookdale in the same 
rock. Specimens from both countries, if accidently mixed, could not 
be separated by any mineral or conchological difference. The species 
which we have identified with those of Europe are the following: 
Delthyris trigonalis, Sowerby, 
cuspidatus, S. 
Producta, analoga, Phillips. 
^Scotia, S. 
* Those species marked with an asterisk have been erroneously referred to the " Primarv 
System." Vide Encyc. Metropolitana; article Geology, p. 575. ""lary 
