CARDIID^. 195 



The transitions between the type and the following subgenera 

 are so gradual that the latter must be regarded as of little value. 



MONODACNA, Eichw., 1838. Hinge with a single tooth. A. 

 Gaspicum, Eichw. (cxvi, 85). 



DiDACNA, Eichw., 1838. Hinge with two teeth. A. Donaci- 

 formis^ Schroter (cxvi, 85 \ 



PROSODACNA, Tournouer, 1882. A.macrodon^T>e^\).. Tertiary; 

 Crimea. 



Hemicardium, Cuvier, 1811. 



Distr. — 28 sp. Tropical. H. cardissa, Linn, (cxvi, 8T). 

 H. hemicardiiini, Linn, (cxvi, 88). 



Shell cordiform, the posterior slope strongly depressed and 

 bordered by a carina ; lunule simple ; cardinal teeth distinct, 

 more or less twisted ; surface radiately ribbed. 



FRAGUM, Bolten, 1798. Anterior side short and truncated; 

 ribs tuberculated. II. imedo, Linn, (cxvi, 89). 



CTENOCARDiA, H. and A. Adams, 1855. Like Fragum, but ribs 

 spinose. H. hystrix^ Brod. (cxvi, 90, 91). 



LUNULiCARDiA, Gray, 1847. Lunule depressed, surrounded by 

 a deep broad channel ; ribs nearly smooth and flattened on the 

 posterior slope. H. retusa, Linn, (cxvi, 92). 



CoNOCARDiUM, Bronn. 



Syn. — Lychas, Stein. Pleurorhynchus, Ph. Lunulocardium> 

 Miinster. Arcites, Martin. 



Distr. — Fossil, 30 sp. U. Silurian — Carb. ; North America, 

 Europe. G. aliforme, Sowb. (cxvi, 93). G. Hibernicum, Sowb. 

 (cxvii, 100). 



Shell equivalve-trigonal, conical and gaping in front, truncated 

 behind, with a long siphonal tube near the umbones ; anterior 

 slope radiately, posterior obliquely striated ; margins strongly 

 crenulated within ; hinge with anterior and posterior laminar 

 teeth ; ligament external. 



The truncated end has usually been considered anterior, a 

 conclusion which seems incompatible with the vertical position 

 and burrowing habits of most free and equivalve shells ; if com- 

 pared with Adacna the large gape will be for the foot, and the 

 long tube siphonal. G. Hibernicum has an expanded keel, like 

 Hemicardium inversuvi. The shell-structure is prismatic-cellular, 

 as first pointed out by Sowerby ; but the cells are cubical, and 

 much larger than in any of the Aviculadse. In Cardium, the 

 outer layer is only corrugated or obscurely prismatic-cellular. 



LiTHOOARDiUM, Desh. Shell triangular, keeled ; anterior side 

 very short ; hinge-teeth 1*2, directed backwards ; posterior lat- 

 erals 2*1; anterior muscular pit minute,, posterior impi-ession 

 large, remote from the hinge. L. cymbulare, Lam., exhibits 

 slight indications of a byssal sinus in the front margins of the 



