244 
PALAEONTOLOGY OF NEW YORK. 
Genus ORTHIDIUM, gen. nov. 
PLATE VII A, FIGS. 22-25. 
1862. Orthis, Billings. Palaeozoic Fossils, vol. i, p. 75, tigs.(j8, a, b, c. 
Diagnosis. Shell very small, transverse, having the external aspect of 
Scenidium. Hinge-line long, making the greatest diameter of the shell. Ped¬ 
icle-valve the more convex; cardinal area moderately high, with a broad open 
delthyrium, strong teeth and inconspicuous dental plates, in all respects like 
the corresponding valve of Orthis calligramma. Brachial valve slightly con¬ 
vex ; cardinal area very narrow; dental sockets well developed, crural plates 
very short and erect, coalesced with the cardinal process, which thus becomes 
a vertical, transverse, subcrescentic plate, at the base of which the shell is 
somewhat excavated; muscular scar quadruplicate. Surface covered with 
radiating striae, which extend over the broad, low sinus and fold in the brachial 
and pedicle-valves respectively. In the former the sinus makes a prominent 
median ridge in the interior of the valve. Shell-structure not determined. 
Type, Orthis gemmicula, Billings. Quebec group. 
Observations. This interesting fossil presents the earliest known phase of 
the development of the cardinal process which characterizes Strophomena, and 
the other streptorhynchoid genera; and it is upon this feature that its 
separation as a distinct genus is rendered necessary. All the features of the 
pedicle-valve are strictly orthoid. 
On the brachial valve the cardinal process instead of being bipartite at its 
crest, and grooved or multipartite on its posterior face, is simple, terminating 
in a rounded apex and filling the delthyrium. 
These features are beautifully shown in Mr. Billings’ original specimens of 
Orthis gemmicula, which have been kindly loaned by the Director of the Geo¬ 
logical Survey of Canada, through Mr. J. F Whiteaves, for the purpose of 
illustration in this volume. No other species referable to the same genus is at 
present known. Orthis gemmicula is from the Quebec group, at Point Levis, 
“ in the upper part of Limestone No. 2” (Billings). 
