258 
PALAEONTOLOGY 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 Vanuxem’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 lamellae 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 Orthotiietes 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 striae. 
Type, Hipparionyx proximus, Vanuxem.^ Oriskany sandstone. 
* Vanuxem also cited two other species of Hipparionyx, H. consimilM and H. similaris, and on page 132 
of his report gives a figure probably intended for one of these species, under the name of H. consimilaris. 
This latter is a large, expanded form of Atrypa reticularis, from the Onondaga limestone. 
