TRENTON LIMESTONE. 
121. 
161. 8. ORTHIS FISSICOSTA ( n. sp .). 
Pl. XXXII. Figs. 7 a , b. 
Compare Orthis calKgramma. Dalman, Hisinger, de Verneuil. 
Orthis actonice. Sowerby in Murchison’s Sil. System, pag. 639, pi. 20, fig. 16. 
Semioval, with the cardinal line somewhat less than the width of the shell; area narrow ; 
foramen narrow, triangular, reaching to the apex ; dorsal valve moderately convex, with 
the beak extending and slightly incurved ; ventral valve moderately convex in the middle, 
and depressed at the sides ; surface marked by angular costa;, which become bifid and trifid 
on the centre or towards the margin of the shell; number of costas about 19 or 20. 
This shell, in its general form, resembles the 0. calligramma ; but it is clearly a distinct 
species, judging from a single European specimen in my possession. The bifid, trifid, and 
even quadrifid costse are an obvious difference, though such a change may take place in 
the costa; of O. calligramma. The costa; are fewer on this species, and more elevated ; the 
ventral valve is convex, while in that species it is flat and slightly depressed in the centre; 
the dorsal valve is less convex in our species, though the specimen is somewhat crushed. 
In the divided character of the costa;, this shell resembles the Orthis actonice cited above ; 
but our specimen is smaller than the one figured by Sowerby, and has a greater number 
of costse (19 on the dorsal and 20 on the ventral valve, while that one has 14). A com¬ 
parison with a single specimen of 0. actonice shows the costa; to be more distant in that 
species than in ours. 
Fig. 7 a. View of the ventral valve, showing a portion of the area and beak of the dorsal valve. 
Fig. 7 b. Profile view of the same, showing the projecting beak of the dorsal valve. The dorsal valve 
has been crushed, so that the profile view does not give a correct idea of its convexity. 
Position and locality. This species occurs with the preceding ones in the Blue limestone 
of the West, but it is unknown to me in New-York. I obtained but a single specimen near 
Cincinnati, showing that the shell is comparatively rare. 
162. 9. ORTHIS TRICENARIA. 
Pl. XXXII. Figs. 8 a, b, c, d, e. 
Orthis tricenaria. Conrad, Proc. Acad. Nat. Sciences, 1843, Vol. i. p. 333. 
Semioval ; length and breadth nearly equal; cardinal line straight, equal to, or greater 
than the width of the shell; area very broad ; foramen long, sublinear ; dorsal valve sub- 
conical, with the apex much elevated and produced, though scarcely incurved ; cardinal 
margins sloping very obliquely from the beak ; ventral valve flat, regularly rounded from 
the extremities of the hinge line ; surface marked with from 16 to 32 rounded simple 
radii, with spaces between equal to the radii. 
The most striking character of this shell, and one which will serve to distinguish it from 
any other Orthis known to me in New-York, is the great elevation of the dorsal valve, and 
remarkably broad cardinal area; the ribs are always simple, continuing distinct quite to 
the apex of each valve, and gradually enlarging towards the base ; the foramen is some¬ 
what linear in perfect specimens, and continues quite to the beak. 
[ Paleontology.] 16 
