304 PROCEEDING'S OF THE NATIONAL MUSEUM. vouxxiv. 
attachment on the lower valve; attached valve moderately convex, 
but varying- considerably in this respect; upper valve ^ slightly con- 
cave, almost conforming to the curvature of the lower valve, so that 
the space between them is extremely thin; surface of both valves 
marked by rather line radiating ribs, about equal in width to the 
interspaces, and many of them branching one or more times before 
reaching the margin, or with short ribs interpolated between them; 
sculpture often equally strong on both valves, but in some cases 
nearly obsolete on the upper valve; internal features as described for 
the genus. 
Average specimens usually measure from 130 to 150 mm. in length 
and about 70 to 75 mm. in greatest breadth. One ovate specimen 
measures 121 mm. in length and 94 mm. in greatest breadth and occa- 
sional examples are more nearly circular in outline. One very large 
triangular specimen collected by Mr. Hill is 166 mm. in length aild is 
expanded in fan-shape toward the base until it is 172 mm. broad. 
The convexity or thickness of the two valves united is seldom more 
than 10 mm. and there is little difference in this respect between young 
and adult shells. 
There is a perceptible, though not great, variation in the coarse- 
ness of sculpture of different individuals, as may be seen by the illus- 
trations. 
The species is associated at most localities with RadioUtes dcwid- 
soni Hill and it is often found attached to that form. This habit led 
Cragin and other members of the Geological Sui'vey of Texas to sup- 
pose that both shells belonged to the same animal which was described 
as Ilipjntntes flahellifer.^ The shell is usually attached by one edge 
of the lower valve near the beak and it often shows a tendency to grow 
around the supporting object. A curious example of this is seen in 
the smaller of Hill's figured types (now preserved in the geological 
department of Johns Hopkins University), which shows a fragment of 
RadioUtes shell nearly half an inch long firmly attached in a deep 
notch that involves both valves of the Chondrodonta. 
Ostrea joannm Choff'at^ from the Cretaceous of Portugal is very sim- 
ilar to this species in form, sculpture, and all other described characters, 
except that the sculpture is considerably coarser. Through the 
courtesy of Prof. Paul Chofl'at I have received a dozen specimens of 
0. joanncB from Monte Serves and Runa, Portugal, since this descrip- 
tion was written. Sections were ^ut across the umbonal region of 
^ Mr. Hill inadvertently described the upper valve as the lower. 
^ This name has a few weeks' priority over Osfrea munsoni, but since it was applied 
to two entirely distinct forms its designation as Hippurites necessitates its restriction 
to the Rudistid portion of the complex species. 
^ Recueil d'etudes paleontologiques sur la faune Cretacique du Portugal, I, p. 34, 
Ostreidae, pi. i, figs. 1-7, and pi. ii, figs. 8-19. Lisbon, 1886. 
