202 HIPPURITID-ffi. 



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. — Pratt. 



Chamostrea, Roissy, 1805. 



Syn. — Cleidothaerus, Stutch., 1829. 



Distr. — 1 sp. New South Wales. G. albida, Lam. (cxvii, 16, 

 11). 



Shell inequivalve, Chama-shaped, solid, attached by the 

 anterior side of the deep and strongly-keeled dextral valve ; 

 umbones anterior, subspiral ; left valv'e 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 b}^ an oblique furrow, the dorsal portion con- 

 sisting of a single lamina with a free margin. 



Family HIPPURITID^. 

 (Order Rudistes, Lamarck.) 



Shell inequivalve, 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. Romer has 

 I'ound 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 destroj^ed by the weathering of the specimens. 

 The principal la.yer in the lower valve of Hippurites is not 

 really veiy different from the upper valve in structure ; the 

 laminre 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 

 3'oung shell. 



