174 PROCEEDINGS OF THE GEOLOGICAL sociETy. [Dec. 1, 
Orthis carinata; shell transversely semi-oval, concavo-convex ; 
hinge-line hardly equal to the width of the shell; hinge-area, on the 
dorsal valve narrow and slightly triangular, with a broad open trian- 
gular foramen; on the ventral valve very narrow. Dorsal valve con- 
vex, with the two sides slopmg away from the middle rib, which is 
larger than the rest, giving it a roof-shaped or keeled form ; covered 
by strong simple plaits, separated by rounded hollows twice the 
width of the plaits; plaits narrow and rounded at top, diminishing 
gradually in size from the middle keel towards the hinge-margin, 
near thirty in all. Ventral valve concave with broad rounded plaits, 
separated by narrow deep furrows, of which the middle one is the 
largest and deepest. 
The surface of both valves is covered by minute, irregular, con- 
centric wrinkles left by the growth of the shell: in old specimens 
these wrinkles form a rough scaly margin to the shell. 
Very common in the beds of the Hamilton group of New York. 
In general form this shell resembles Orthis semicircularis of Kich- 
wald, as figured by M. de Verneuil*; but in its plication it is more 
like O. calligramma. It is however very distinct, and cannot be con- 
founded with any other species. 
(8) Orthis parva has been confounded with the O. elegantaie by 
all English geologists under the name of O. canalis: the differences 
between them have been clearly pointed out by M. de Verneuil+. 
In O. elegantula a narrow stripe down the middle of each valve 
is occupied by finer strize than those which cover the rest of the 
shell: by this character it is easily recognized ; besides this distine- 
tion it has the dorsal valve deeper than the other species. O. parva 
has its strisee somewhat fasciculated, those strize which have had a 
common origin running together in a sort of cluster. O. argentea of 
Hisinger has this last character strongly marked, and is probably the 
young of the same species. O. parva and elegantula are found in 
different formations, both in this country, in Russia and in the United 
States ; the former belonging to the Lower Silurian beds, the latter 
to the Wenlock series. 
(9) PENTAMERUS GALEATUS.—The American specimens do not 
exactly agree with any of the European varieties of this most variable 
species, and yet they present no characters which justify our sepa- 
rating them as a distinct species. The principal difference consists 
in a greater flatness of form, caused in part by the slighter elevation 
of the ventral valve, and partly by the lowness of the mesial ridge on 
the dorsal valve. The ribs are nearly equal in size, about fifteen in 
number, and commence near the beak, so as to cover the greater part 
of the shell: there are also specimens on which the ribs are very 
faint, and the shell is nearly smooth. The interior characters agree 
with those of our Upper Silurian specimens: the two plates which 
enclose the middle chamber of the ventral valve are entirely detached 
along their bases, and only meet at the beak ; in this respect they 
differ from a smaller species found in the Eifel, im which the two 
plates are united together along the whole of their base, so as to ap- 
* Russie, vol. il. t. 13. f. 12. t+ Jb. vol. ii. p. 188. 
——- = ee 
