180 CORBULID.E. 



primary tooth In one or both valves, with correspon cling 

 socket and Hgamental pit beside it. Ligament small, in- 

 terior. Muscular impressions slightly marked, united by a 

 pallial one with a very slight sinus. 



Animal short, with very short united siphonal tubes. 

 Orifices fimbriated. Mantle closed, except in front, where 

 there is an opening for a bony, narrow, thick foot of con- 

 siderable dimensions. Anal siphon with a conspicuous 

 tubular membrane. Labial tentacles slender. 



This is one of those genera which have diminished with 

 the course of time. CorhulcE were abundant in the European 

 seas during the earlier part of the tertiary epoch, and even 

 before. They are now reduced to a very few species. The 

 genus has more representatives at present in tropical seas. 



The microscopic structure of the shell has been ex- 

 amined by Dr. Carpenter. He finds the outer layer to 

 consist of large fusiform cells ; the inner to be nearly homo- 

 geneous. " At the lines of junction of the successive ad- 

 ditions to the margin is a yellow layer, probably owing its 

 colour to an intermixture of horny matter, such as might, 

 if poured out upon the surface, have formed a periostracum. 

 Li this layer there is a very definite and beautiful cellular 

 arrangement, the cells being in some parts polygonal, and 

 having their edges in contact, whilst in others they are 

 rounded and isolated." — {Jteport^ p. 104.) 



0. NUCLEUS, Lamarck, 



Rather obliquely sul)triangular, very solid, not polished, sub- 

 inequilateral, whitish, with the larger valve sometimes rayed with 

 pale crimson, and the smaller one always covered with a stiff 

 umberbrown epidermis : ventral edge arcuated, ascending and 

 straighter posteriorly ; dorsal edges moderately and subcqually 

 sloping, and nearly straight. Valves excessively unequal, vcntri- 



