CLASiS I 



PELECYPODA 



481 



Formeily the groiip was referred to tlie most diverse connections, such as Biachiomds, Corals 

 Cirripedes, etc., or placed in a special class by itself. ' 



The luajority of Rudislids occur gregariously in hirge nuuibers, sometinies filling entire 

 beds ; they are often found m their natural position, standing vertically on the apex of the 

 attached valve. Notwithstanding their abundance, it is extremely difficult and often irapos- 

 sible to separate the two valves and expose the interior, hence the hinge of many species is 

 still only inij>erfectly known. 



Family 13. Radiolitidae Gray (emend.). 



Shell sabstance wüh the external layer thick, prismatic ; the internal thin, cellulo- 

 crystalline (frequently destroyed in fossilisation) ; valves very unequal, the ligamentary 

 sitbsidence usually marhed ; free valves wüh two projections and two somewhat irregulär 

 myophores ; fixed valve with one myophore and two sockets ; summit of the valves suh- 

 marginal in the young, subcentral in the adult. Cretaceous. 



a, h, RadioUtcs {IHradiolites) cornu-pasturis d'Orb. Middle Cre- 

 taceous (Oarentonian) ; Pyles, near Perigeux. a, Shell with closed 

 valves. B, C, The two more tinely ribbed vertical bands. h, Interior 

 of larger valve, viewed from above. a, a', Adductor scars ; m, Pallial 

 line ; n, Space occupied by soft jmrts ; x, Empty space botween the 

 sockets. 1/2 (after Bayle). c, Opercular valve of Ä. bournoni (\)esn\.). 

 Upper Cretaceous (Dordonian) ; St. Mametz, Dordogne. a, Anterior ; 

 a', Posterior myophore ; c, c', Anterior and posterior processes of 

 clithrum. 1/3 (after Bayle). 



Fi(i. 779. 



Ujtper 

 Shell 



Spliaerulites angeiodes Lam 

 Cretaceous ; Gosau, Austria. 

 with closed valves, Vi- ^. Opercular 

 valve, 1/1. A, Sinus of hinge; a, a', 

 Myophores ; c, c', Processes of clithnim. 



Radiolites (Lani.) Bayle (Biradiolites d'Orb.) (Fig. 778). Lower valve conical, 

 erect, elongated, vertically ribbed, or made iip of siiccessive layers ; usually with two 

 somewliat smooth bands exteiiding from the apex to the upper margin, which are 

 supposed by Douville to indicate the position of siphonal orifices ; outer layer very 

 thick, composed of large polygonal cells or liollow prisms (Fig. 780). Upper valve 

 operculate, flat or conical, with central or eccentric umbo. The clithrum is formed by 

 two vertically projectiiig striated processes (Fig. 778, c, c) fitting into sockets near 

 the outer wall of the fixed valve; next to and outside of the sockets are two large, 



VOL. I 



2 I 



