202 HIPPURITIDA. 
the body-chamber from 2 to 7 diameters; specimens measuring 
a yard across may be seen on the cavernous shores of the islets 
near Rochelle.—Prartt. 
CuAMostTREA, Roissy, 1805. 
Syn.—Cleidotherus, Stutch., 1829. 
Distr.—1 sp. New South Wales. (C. albida, Lam. (exvii, 16, 
17); , 
Shell inequivalve, Chama-shaped, solid, attached by the 
anterior side of the deep and strongly-keeled dextral valve ; 
umbones anterior, subspiral; left valve flat, with a conical 
tooth in front of the cartilage ; cartilage internal, with an oblong, 
curved ossicle; muscular impressions large and rugose, the 
anterior very long and narrow; pallial line simple. 
Animal with mantle-lobes united by their extreme edge 
between the pedal orifice and siphons; pedal opening small, 
with a minute ventral orifice behind it; siphons a little apart, 
very short, denticulated; body oval, terminating in a small, 
compressed foot; lips bilobed, palpi disunited, rather long and 
obtusely pointed; gills one on each side, large, oval, deeply 
plaited, prolonged in front between the palpi, united posteriorly ; 
each gill traversed by an oblique furrow, the dorsal portion con- 
sisting of a single lamina with a free margin. 
Famity HIPPURITIDA. 
(Order Rudistes, Lamarck.) 
Shell inequivalye, unsymmetrical, thick, attached by the 
right umbo; umbones frequently camerated; structure and 
sculpturing of valves dissimilar; hinge-teeth 1:2; adductor 
impressions two, large, those of the left valve on prominent 
apophyses. 
The shells of this extinct family are characteristic of creta- 
ceous strata, and abound in many parts of the Peninsula, the 
Alps, and Eastern Europe, where the equivalent of the Lower 
Chalk has received the name of “ Hippurite limestone.” They 
occur also in Turkey and in Egypt, and Dr. F. Rimer has 
round them in Texas and Guadaloupe. The structure of these 
shells has been fully described in the Quarterly Journal of the 
Geological Society of London. In all the genera the shell con- 
sists of three layers, but the outermost, which is thin and com- 
pact, is often destroyed by the weathering of the specimens. 
The principal layer in the lower valve of Hippurites is not 
really very different from the upper valve in structure; the 
lamin are corrugated, leaving irregular pores, or tubes, parallel 
with the long axis of the shell, and often visible on the rim. 
The umbo of the upper valve of Radiolites is marginal in the 
young shell. 
