ORDER III TELEODESMACEA 403 
725). Right valve conical or twisted, attached; left valve convex, with incurved 
beak ; ligament as in Chama. Shell structure like Caprina, but the free valve with 
canals in the middle layer; the walls of the canals bifurcate outward, forming in 
section a fringe of peripheral minor channels (Fig. 725, C). Cenomanian and Turonian. 
Caprinula, VOrb. (Chaperia, Mun.-Chalm.), (Figs. 726,727). Right valve elongated, 
attached, conical, or incurved ; left smaller, gyrate ; both with canal system, the peri- 
pheral canals smaller; hinge as in Caprina. Cenomanian and Turonian, especially 
in Portugal, Sicily, and Texas. 
? Ichthyosarcolites, Desm. (Caprinella, @Orb.) Cretaceous. Caprotina, VOrb. (Fig. 
728). Canals obsolete, replaced in some species by cavities. Neocomian to Turonian. 
Coralliochama, White. Right valve conical, elongated, attached ; left smaller, with 
incurved beak; anterior cardinal tooth buttressed, strong; posterior cardinal weak ; 
canals as in Plagioptychus, bounded within by a coarsely cellular layer; lower valve 
with a prismatic outer and laminar inner layer, separated by an intermediate cellular 
stratum. Cretaceous ; California. 
Superfamily 6. RUDISTAE. Lamarck. 
Chamacea in which the spirality of the valves has been lost, the area and ligament 
vertically submerged, and the dorsal margins recurved over them so as to bring the ligament 
into a sub-central position above the teeth but far below the dorsal margin, where it finally 
becomes obsolete. The teeth, no longer forming a hinge but rather a clithrum, specially 
modified for the vertical motion of the operculiform left valve, in which rotation is prevented 
by the projection of the modified teeth into deep sockets in the fixed valve ; the latter conical, 
thick ; pallial line simple, enclosing the whole cavity ; shell structure specialised in two 
very different layers ; sessile, marine. 
The prisms of the outer shell layer are parallel to the long axis of the valve, and are cut 
at right angles by numerous tabulae, which, together with the upper margin, often bear 
impressions of radial vessels. The laminae of which the inner layer is composed are often 
separated by cavities which recall the septa of Cyathophylloid corals, or those cavities found in 
some oyster shells. In Hippurites the outer layer is traversed by a complex of canals. The 
Rudistae are the most peculiarly modified ofall Pelecypods. Their relationship to the Chamidac 
through Monopleura and Caprotina was first recognised by Quenstedt, and afterwards con- 
firmed by Woodward, Bayle, Zittel, Munier-Chalmas, Douvillé, and others. Formerly the 
group was referred to the most diverse connections, such as Brachiopods, Corals, Cirripedes, 
etc., or placed in a special class by itself. 
The majority of Rudistids occur gregariously in large numbers, sometimes filling entire 
beds ; they are often found in their natural position, standing vertically on the apex of the 
attached valve. Notwithstanding their abundance, it is extremely difficult and often impos- 
sible to separate the two valves and expose the interior, hence the hinge of many species is 
still only imperfectly known. 
Family 13. Radiolitidae. Gray (emend.) 
Shell substance with the external layer thick, prismatic; the internal thin, cellulo- 
crystalline (frequently destroyed in fossilisation) ; valves very unequal, the ligamentary 
subsidence usually marked ; free valves with two projections and two somewhat irregular 
myophores ; fixed valve with one myophore and two sockets; summit of the valves sub- 
marginal in the young, sub-central in the adult. Cretaceous. 
Radiolites (Lam.), Bayle (Biradiolites, d’Orb.), (Fig. 729). Lower valve conical, 
erect, elongated, vertically ribbed, or made up of successive layers; usually with two 
somewhat smooth bands extending from the apex to the upper margin, which are 
supposed by Douvillé to indicate the position of siphonal orifices ; outer layer very 
thick, composed of large polygonal cells or hollow prisms (Fig. 731). Upper valve 
operculate, flat, or conical, with central or eccentric umbo. Clithrum formed. by 
