244 PALAEONTOLOGY OF NEW YORK. 



Genus ORTHIDIUM, gen. nov. 



PLATE VII A, FIGS. 22-25. 



1862. OrtMs, Billings. Paljeozoic Fossils, vol. i, p. 75, figs. 8, 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 caUigramma. 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). 



