222 
THE GEOLOGIST. 
where it forms small auriculate expansions. The area in both valves is narrow, 
and divided in the ventral one hy 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 widtli. The interior is unknown. 
Of this elegant species I am acquainted with but a single example, which 
was found at Gare, m Lanarkshire, at two hundred and thirty-nine fathoms 
below "Ell coal." Its identification with C. Buchiana is given upon the 
authority of Prof, de Koninck. It appears to be lilcewise rare in Belgium. 
Eamily Crauiad^. 
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 (V?.). 
V p. Ventral adjuster (H.) 
dp, dorsal adjuster (H.) = protractor sliding muscle (W.). 
p «, (?) Anterior extremity of dorsal adjuster (H.). 
bp. Brachial muscle, posterior extremity (H.) = retractor sUding muscles (W.). 
b. Brachial muscle, anterior extremity (H.) = retractor shding muscles (W.). 
-n. Mesenteric muscle, destined probably to draw the ahmentaJy tube backwards (? H.) 
0, Ovarian (?) ; m, granulated margin. 
rarely free, but genex'ally attached to marine bodies by the umbo (when such 
does exist), or by the entire surface of the lower or ventral valve ; and it is 
from this circumstance that the ventral or attached valve varies so much in 
shape and sculpture. The upper or dorsal valve is always more or less limpet- 
shaped, with a sub-central vertex, the surface being smooth or variously 
sculptured by concentric or radiating striae, or ribs, some also possessing a 
spiny investment. There exists no articulated hinge, the valves being kept 
in place by a peculiar disposition of its muscles ; and although the animal 
