ISOCARDIID^. ' 189 



horizontally extending flexuous groove into which the infero- 

 anterior cardinal tooth of the left valve fits, the supero-posterior 

 cardinal of this valve is moderately prolonged, single or indis- 

 tinctly bifid. 



ANi80CARr)iA,Munier-Chalmas,1863. Surface radiately marked, 

 umbonal ridge not angular. Cretaceous, Eocene. Type, V. 

 elegans, Munier-Chalmas (cxv, 54-56), Kimmeridge clay, Havre. 



VENiLiCARDiA, Stol., 1870. Shell, of large size, strong and 

 thick, the supero-posterior cardinal teeth are usually more or 

 less bifid, the one in the right valve with a very eas}^ curve at 

 the anterior end, the antero-inferior cardinal teeth of both valves 

 are long, flexuous, and their posterior ends are in both cases 

 strongly thickened and tubercular. Jurassic, Cretaceous, Ter- 

 tiary. Tj'pe, V. arcotica, Stolicz. Cretaceous ; India. V. cordi- 

 formis, d'Orb. (^cxv, 57). 



Family ISOCARDIIDiE. 



Shell cordiform or transversely oblong, ventricose, sometimes 

 carinated ; beaks sometimes subspiral ; two cardinal and two 

 lateral teeth in each valve, the anterior lateral tooth occasionally 

 obscure or rudimentary ; muscular impressions narrow ; pallia! 

 line simple. 



IsocARDiA, Lamarck, 1799. 



Etym. — Isos^ like, cardia, the heart. Heart-cockle. 



Syn. — Glossus and Glossoderma, Poll, 1791. Bucardium, 

 Muhlfeldt. Tychocardia, Romer. 



Distr. — 5 sp. Britain, Mediterranean, China, Japan. Fossil, 

 90 sp. Trias — ; United States, Europe, South India. I. cor, 

 Linn, (cxiv, 53-55). 



Shell cordate, ventricose; umbones distant, subspiral; liga- 

 ment external; hinge-teeth 2-2; laterals 1 — 1 in each valve, the 

 anterior sometimes obsolete. 



Animal with the mantle open in front; foot triangular, 

 pointed, compressed ; siphonal orifices close together, fringed ; 

 palpi long and narrow ; gills very large, nearly equal. 



The heart-cockle burrows in sand, by means of its foot, leaving 

 only the siphonal openings exposed. — Bulwer. 



The Isocardia-shaped fossils of the old rocks belong to the 

 genera Cardiomorpha and Isoarca ; many of those in the 

 Oolites to Ceromya. Casts of true Isocardia have only two 

 transverse dental folds between the beaks, and no longitudinal 

 furrows. 



MIOCARDIA, H. and A. Adams, 1856. Shell without epidermis; 

 beaks strongly curved, spiral ; posterior umbonal slope cari- 

 nated ; surface concentrically ridged. I. MoUkiana, Chemn. 

 (cxiv, 56). 



OARDiODONTA, StoL, 1867. Shell cordiform, inflated, with prom- 



