258 PALEONTOLOGY OF NEW YORK. 



to Orthis hipparionyx, and by this designation the species has become generally 

 known. It will, however, be necessary to return to both Van0xem's generic 

 and specific terms. 



Diagnosis. Shell large, subhemispherical. In youth the pedicle-valve is 

 very slightly convex, but at maturity it is depressed, or concave, over the pallial 

 region. The brachial valve is always very convex. Marginal outline of the 

 valve subcircular. On the pedicle-valve the hinge-line is straight but short, the 

 cardinal area low, and the beak retrorse. The delthyrium is broad, and covered 

 by an imperforate convex deltidium. The teeth are moderately large and are 

 supported by lamellas which extend to the bottom of the umbonal cavity, and 

 are produced into strong ridges entirely surrounding the muscular area. This 

 area is very large, having the structure of that in extreme examples of Rhipi- 

 DOMELLA, such as Orthis musculosa, and is composed of broad, flabellate diductors 

 enclosing an elongate or cordate adductor impression There is a low median 

 septum in the umbonal cavity, separating the arms of the cardinal process of the 

 opposite valve, but it is quite short, disappearing at the adductor scar, though 

 sometimes reappearing in front of it. 



In the brachial valve there is no cardinal area. The cardinal pro- 

 cess has essentially the same structure as in Orthothetes and Derbya, but 

 is very high, its two branches completely traversing the umbonal cavity of 

 the opposite valve ; it is supported by a short, median septum, and laterally 

 by strong crural plates which extend for a short distance along the margins 

 of the muscular impressions ; these are usually quite faint and undefined, 

 occupying a much smaller area than in the pedicle-valve, and leaving arbores- 

 cent markings as in some species of Stropheodonta. On the interior of both 

 valves the margin is sharply pectinated, or crenulated, the crenulations on the 

 brachial valve extending to the base of the cardinal process, and in the 

 pedicle-valve extending for a considerable distance on the cardinal area. Sur- 

 face of both valves covered with fine sharp radiating strise. 



Type, Hipparionyx proximus, Vanuxem.* Oriskany sandstone. 



* Yanuxem also cited two other species of Hipparionyx, H. coiunmills and H. tiiiiiilaris, and on page 132 

 of his report g-ives a figure probably intended for one of these species, under the name of IT. cinisimilwis. 

 This latter is a large, expanded form of Atrypa reticularUi, fi'om the Onondaga limestone. 



