70 
Horizon and Localities . — According to Mr. J. Hall this species is not 
rare in the calcareous shales of the Hamilton Group of the regions bordering 
Lakes Cayuga, Seneca, and Canandaigua, and many other localities. The 
specimens collected by Mr. Clarke occur in a mottled phtanite at Ivempsey. 
Genus — OHTHIS, Lalman. 
1. Orthis interltneata, Soioerhy. 
OrthlS interlineata , Sowerby, 1840, Traus. Geol. Soc. London, V (2), pi. 53, fig. 11, and 
pi. 54, fig. 14. 
,, ,, J. Phillips, 1841, Pal. Loss. Cornwall, p. 63, pi. 26, fig. 106. 
,, 'parallela, Idem, 1841, Hid., p. 64, pi. 26, fig. 109. 
,, intevlineata , McCoy, 1855, Brit. Pal. Boss., p. 384. 
,, ,, T. Davidson, 1865, Mon. Brit. Dev. Brach., p. 91, pi. 17, fig. 18-23. 
Shell slightly transverse, suhelliptic, thin ; hinge line shorter than 
the transverse diameter of the shell ; ventral valve slightly convex, with only 
a narrow area, a little-marked beak, and a very small deltidium ; dorsal valve 
shallower than the opposite one, slightly sinuated in the middle and provided 
with a linear area. 
The external surface of both valves is ornamented by some number of 
small radiating ribs, slightly angular, commencing at the beak and proceeding 
in a straight line to the margins ; between these ribs, which are the thickest, 
are developed some other thinner ones, which are at variable distances from 
the hinge, thus augmenting considerably the number of those that extend to 
the margin. They are sometimes crossed by faint concentric growth lines. 
Limensions . — Length about eleven millimetres ; wudth twelve 
millimetres. 
Horizon and Localities . — A rather large block of compact black 
limestone from Yarradong contains some number of examples of this species, 
which docs not appear to he uncommon near Petherwin in England, and 
Charlemont, near Givet, in Prance. 
