302 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxiv. 
is referred to this g-enus. Authentic examples of Osti'ea joannce also 
show the characteristic hinge structure of Choiidrodonta as seen in 
cross section. The collections referred to, except the types of Hill 
andCragin, are in the United States National Museum, and this notice 
is published here by permission of the Director oi the United States 
Geological Survey. The descriptions of the genus and of the two 
American species follow. 
CHONDRODONTA, new genus. 
Shell of rather large size, sessile, ostreiform, attached by the left (?) 
valve; texture, subnacreous; hinge plate greatly elongated, without 
proper teeth, but with a long chondrophore in each valve a little back 
of the median line, extending from the beak across the hinge plate 
and projecting considerably beyond it into the body cavit}^ the chon- 
drophore of the attached valve forming the overhanging anterior wall 
of a deep, narrow groove into which is received the chondrophore of 
the upper valve in the form of a rather thin oblique lamella, whose 
free edge is slightly curved upward. Near their umbonal ends the 
chondrophores are nearly in contact, but toward their other extremities 
they are separated by a space of from 1 to 3 millimeters (according to 
the size of the shell). This space must have been filled by the resilium, 
which was evidently attached to the under side of the chondrophore in 
the lower valve and to the upper side of that in the free valve, the 
whole forming an interlocking arrangement that could have allowed 
very little motion of the valves, and is effective even after the resilium 
has disappeared, for the two valves are almost invariably found firmly 
united. 
Pallial line remote from the margin, as in Pectinidae, Spondylidae, 
etc., usuall}^ not ver}^ distinctl}^ marked; adductor muscle doubtless 
single, though its faint impression has not been positively recognized; 
lower valve, moderately convex; upper valve, flat or concave, so that 
the bod}" cavity is very shallow; surface, either with radial, irregularly 
dichotomous plications, or nearly smooth, with only concentric growth 
lines. 
Tyx)e. — Ostrea Diunsoni Hill, from the Edwards limestone of Texas; 
also represented by Chondrodonta glabi'ci^ new species, from probably 
the upper part of the Glen Rose limestone near Kerrville, Tex. 
European representatives of the genus are Ostrea joannce from Portu- 
gal, probably (). aff. niunmnl (Hill) Boehm, and possibly Terqiiemla 
forojxdiensh Boehm from the southern Alps, all of which have been 
referred to the Cenomanian, though varjing opinions have been held 
as to their exact position in the Cretaceous, the latest reference of O. 
joaniHV being to the Turonian and of the Italian form to the Albian 
or Gault. 
