222 



THE GEOLOGIST. 



where it forms small auriculate expansions. The area in both valves is narrow, 

 and divided in the ventral one by a small fissure covered with a pseudo- 

 deltidium. The dorsal valve is concave, and follows the curves of the opposite 

 one. Externally each valve is ornamented with about fourteen simple ribs, 

 with interspaces of almost equal breadth. No cardinal tubes could be observed 

 or were preserved in the specimens under description, which measured five 

 lines in length by six and a-half in width. The interior is unknown. 



Of this elegant species I am acquainted with but a single example, which 

 was found at Gare, in Lanarkshire, at two hundred and thirty-nine fathoms 

 below "Ell coal." Its identification with C. BucUana is given upon the 

 authority of Prof, de Koninck. It appears to be likewise rare in Belgium. 



Eamily Ceaniad^. 



Genus Crania. Retzius. 1781. 



The shells composing this remarkable and widely-spread genus vary much in 

 shape, although not much difference has taken place in this respect in time, for 

 some Palaeozoic species can hardly be distinguished from more recent and even 

 living types. They are all marginally more or less circular or sub-quadrate. 



Ventral, or Attached Valve. Dorsal, or Free Valve. 



Lign, 9. — Crania Ignahergensis (var.). Cretaceous. 



a, Occlusor (Hancock) = anterior adductors (Woodward) . 

 r, Divaricator (H.) = posterior adductor (W.). 



V p, Ventral adjustor (H.) 



dp, dorsal ad.justor (H.) = protractor sliding muscle (W.). 

 p s, (?) Anterior extremity of dorsal adjustor (H.). 



b p, Brachial muscle, posterior extremity (H.) = retractor shding muscles (W.)- 



b, Brachial muscle, anterior extremity (H.) = retractor shding muscles (W.). 



n, Mesenteric muscle, destmed probably to draw the alimentary tube back^'ards (? H.) 

 o, Ovarian (?) j m, gi-anulated margin. 



rarely free, but generally attached to marine bodies by the umbo (when such 

 does exist), or by the entire siu't'ace of the lower or ventral valve ; and it is 

 from this circumstance that the ventral or attached vah^e varies so much in 

 sha])c and sculpture. The upper or dorsal valve is always more or less limpet- 

 shapcnl, with a sub-central vertex, the sui"face being smooth or variously 

 sculpt II rod by concentric or radiating striae, or ribs, some also possessing a 

 spiny investment. There exists no articulated hinge, the valves being kept 

 iu place by a peculiar disposition of its muscles ; and although the animal 



