72 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



contrary to the forms studied by me. This has already been mentioned above (ef. 

 under the heading of Ch. pellucida). The account of the coecal appendage in 

 question is quite enigmatical. 



In all the forms of the genus Pseudochama the appendage named is entirely 

 absent. This fact is noteworthy and vvill be useful in verifying the generic distinc- 

 tion betvveen Chama and Pseudochama that we have concluded to exist on account 

 of the characteristics of the shell. 



Of the liver canals there is usually one larger anterior one, descending into 

 the lower part of the liver and some smaller ones debouching on each side of the 

 stomach. The large chief liver duet contains masses of nutritive particles. The 

 same conditions prevail in Pseudochama as in Chama, but the rule has exceptions, 

 inasmuch as some species show an aberrant arrangement. Thus in Pseudochama 

 exogyra, Chama gryphoides and Ch. lazarus the generally single principal liver duet 

 has been divided into two, one at each side of the stomach. There is also some 

 variation in the ramification and the debouching of the other duets, but within the 

 same species the perfection of the conditions mentioned seems to be constant, since 

 they are found to be similar in two specimens of some species which were examined. 



Even the bipartition of the duodenal portion of the alimentary canal is quite 

 similar in Chama and Pseudochama. The narrower furrow, serving as the digestive 

 portion, always lies on the right side, and the wider one on the left. There are 

 some slight aberrations in this respect, inasmuch as Pseudochama exogyra and P. 

 pusilla have the narrow furrow situated more to the front of the larger one. 



The smaller of the two cavities of the duodenum is interpreted by Grieser 

 (1913) as representing an intestinal portion and the larger a sort of crystalline sheath, 

 which has not yet been separated into a special pouch as in a number of Lamelli- 

 branchs (such as Mya, ef. Vlés 1909; Donax, Barrois 1889; Tagelus, Bloomer 1906; 

 Psammobia, Bloomer 1910). Similar conditions as in the Chamidae prevail in Area 

 (Matthias 1914), Mytilus (List 1902), Cardium (Johnstone 1899), Anodonta (Lang 

 1900, Schwanecke 1913) and other forms. In Pseudochama jmsilla signs of separa- 

 tion may be observed, as here the crystalline portion descends freely in the form 

 of a short sac. Grieser has found the crystalline style in some of his specimens 

 of Ch. pellucida, but I have never seen any trace of a secretion in the corresponding 

 portion of the duodenum. 



In the lower end of the duodenum both portions pass into the intestine proper; 

 this is performed by a gradual dilatation of the intestinal furrow and a simultaneous 

 diminishing of the other one, their separation being previously indicated by the 

 appearance of a typhlosolis. 



When bending into the intestine the alimentary canal may form a simple curve 

 backwards (Chama jukesi, imbricata, lazarus) or describe a flexure or coil towards 

 the front (Pseudochama pusilla, exogyra, ferruginea, Chama gryphoides), or a coil 

 towards the back (Chama coralloides, Echinochama arcinella). A median thickening 

 of the intestine or a colon exists in Chama jukesi and Pseudochama exogyra, but is 

 not observed in P. pusilla, P. cristella, Echinochama and Ch. gryphoides. 



