240 
PALAEONTOLOGY OF NEW YORK. 
Delthyrium covered by a narrow, convex plate; the presence of a foramen not 
determined. On the interior the dental lamellae are widely separated, and 
descend along the umbonal cavity for a short distance vertically, thence 
bending sharply inward and meeting at a low angle in the median line; thus 
forming, with the deltidium, a conspicuous subrostral vault. This inner spoon¬ 
shaped plate, spondylium, is supported by a stout median septum, and two 
smaller lateral septa, which meet it at the lines of angulation; the former of 
these extends for the entire length of the plate, while the latter is free from 
the accessory septa near its anterior edge. The umbonal cavity of the valve 
is thus divided into five chambers, and in the lateral chambers there is still 
another septum, lower than the rest and not extending to the spondylium. 
The brachial valve is shallow and depressed-convex, with a narrow cardinal 
area. The delthyrium is very broad with a. partially developed covering, the 
dental sockets are widely separated, the crural plates narrow and nearly paral¬ 
lel to the hinge-line The cardinal process is simple, linear and quite promi¬ 
nent, and at its union with the crural plates is a .subtriangular thickening 
which is supported by a low median septum. Surface covered with fine, 
elevated, radiating striae, without evidence of median fold and sinus. 
Type, Hemipronites apicalis, Whitfield (not Orthis ? apicalis, Billings). Calcif- 
erous beds. 
The closest relations of this sub-genus are with Clitambonites (= Orthisina), 
being chiefly distinguished by the multipartite umbonal cavity of the pedicle- 
valve. The typical species is from the fauna of the Calciferous sandstone of 
Fort Cassin, Vermont. With this species Mr. Whitfield compares the Orthis? 
apicalis of Billings, from the “Quebec group” (Point Levis Limestone, No. 1),* 
but after examining the original specimens of both species, and with consider¬ 
able additional material from Fort Cassin, there can be no hesitation in pro¬ 
nouncing the two forms as quite distinct, not only in internal structure, but 
also in external characters. 
* Palaeozoic Fossils, p. 331, fig'. 291 a. 
