CLASS I 



PELECYPODA 



481 



Formerly the group was referred to the most diverse connections, such as Brachiopods, Corals, 

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



Tlie majority of Rudislids 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 aud often impos- 

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

 still only imj^erfectly knoAvn. 



Family 13. Radiolitidae Gray (emend.). 



Shell sabstance with the exteriud layer thick, prismatic; the internal thin, cellulo- 

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

 subsidence usually marked ; free valves with tivo projections and two somewhat irreyular 

 myophores ; fixed valve with one myopihore and two sockets ; summit of the valves sub- 

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



a, i, Radiolites (BiradioUtes) cornu-pastoris d'Orb. Middle Cre- 

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

 \-alves. B, C, The two more finely ribbed vertical band.s. b, Interior 

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

 line ; u, Space occupied by soft jiarts ; x, Empty space between the 

 sockets. 1/2 (after Bayle). c, Opercular valve of iJ. hournoni (Desm.). 

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

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

 clithrum. 1/3 (after Bayle). 



Spliaendites angeiodes Lam. Ujiper 

 Cretaceous ; Gosau, Austria, a, Shell 

 with closed valves, 1/1. h, Opercular 

 valve, Vi- -*!) Sinus of hinge; a, a', 

 Jlyophores ; c, c', Processes of clithrum. 



Radiolites (Lam.) Bayle [BiradioUtes d'Orb.) (Fig. 778). 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 Douville to indicate the position of siphonal orifices ; outer layer very 

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

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

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

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

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