236 



MANUAL OF THE MOLLUSCA. 



CRANIA, Retzius. 



Etym. Kraneia, capitate. Type, Anomia craniolaris, L. 



Ex, C. Iguabergensis, PL XV. fig. 30. C. anomala, figs. 157159. 



Syn. Criopus, Poll. Orbicula {anomala) Cuvier, = O. Norvegica, Lara, 



Shell smooth or radiately striated ; umbo of dorsal valve sub-central : of 

 ventral valve sub-central, marginal, or prominent and cap-like, with an ob- 

 scure triangular area traversed by a central line. 



The large muscular impressions of the attached valve are sometimes 

 convex, in other species deeply excavated; those of the upper valve are 

 usually convex, but in C. Parisiensis the anterior (central) pair are deve- 

 loped as prominent diverging apophyses. In C. tripartita, Minister, the 

 nasal process divides the fixed valve into three cells.* 



C. Ignabergensis is equivalve, and either quite free or very slightly 

 attached. C. anomala is gregarious on rocks and stones in deep water, both 

 in the North Sea and Mediterranean (4090 fathoms, living ; 150 fms. 

 dead ; Forbes) : the animal is orange- coloured, and its ]abial arms are thick, 

 fringed with cirri, and disposed in a few horizontal gyrations (fig. 159.) 



Distr. 5 sp. Spitzbergen, Brit. Medit. India, New S. Wales. 150 fms> 



Fossil, 28 sp. L. Silurian . Europe. 



C. antiquissima, Eichw. (Pseudo-crania M'Coy) is free, and has the inter- 

 nal border of the valves smooth ; the branchial impressions blend in front. 

 Spondylobolus craniolans, M'Coy, is a small and obscure fossil, from the 

 L. Silurian shale of Builth. The upper valve appears to have been like 

 Crania, the lower to have had a small grooved beak, with blunt, tooth-like 

 processes at the hinge-line. 



Fig. 159. Crania.^ 



Fig. 160. Discina.% 



* M. Quenstedt has placed the Oolitic Cranias in Siphonaria! 



t Dorsal valve with the animal, seen hy removing the mantle. 



J The animal as seen on the removal of part of the lower mantle-lobe, the ex- 

 tremities of the labial arms are displaced forwards, in order to show their spiral 

 terminations: p, is the expanded surface of the pedicle; the mouth is concealed by 

 the overhanging cirri. The mantle-fringe is not represented. 



