414 MOLLUSCA SUB-KINGDOM vi 



Isocardia, Lam. (Figs. 756, 757). Inflated, smooth, or concentrically striated ; 

 beaks distant, much produced, prosogyrate. Jura to Recent. 



B 



FIG. 750. 

 Isocardia striata, d'Orb. Portlandian ; Cirey, Haute-Marne (after Loriol). 



Meiocardia, Adams. Keeled, concentrically ribbed. Tertiary and Recent. 

 ? Clisocolus, Gabb. Cretaceous ; North America. 



n 



FIG. 757. 

 Hinge of Isocardia lumdatu, Nyst. Crag; Antwerp. 



Family 30. Callocardiidae. Ball. 



Shell as in the Isocardiidae, but with low and inconspicuous beaks, the valves more elong- 

 ated, and the lunule delimited by a sharp groove; cardinal formula j>T/)T~ fA- Tertiary 

 and Recent. 



Oallogonia, Dall. Pallial line deeply sinuated, and a distinct anterior lateral close 

 to the cardinal teeth. Recent ; abyssal. 



Callocardia, Adams. Rounded or Tapetiform, compressed, or not inflated, with 

 low inconspicuous, 11011 -gyrate beaks. Eocene and Recent. 



D. TELEODONTA. 



The most perfected type of modern teeth, to which, in addition to the normal (10101) 

 cardinal series of the ordinary Teleodesmacea, there is added in the most specialised types 

 (Veneridae, Mactridae) either a roughened area (Venus), a series of extra cardinals 

 (Tivela), or accessory lamellae (Mactra), rendering the hinge more efficient, or complicated. 

 The hinge characters of the less specialised forms hardly differ from the Diogenodonta, 

 but they are grouped here on account of their obvious affinities, as shoivn by other 

 characters. 



Superfamily 12. VENERACEA. Menke. 



Teleodonts with normal gills united to form a complete anal chamber, the mantle 

 lobes free behind the siphonal region, sub-equal adductors, an external parivincular liga- 

 ment seated in a groove, and the shell substance densely cellulo-crystalline with incon- 

 spicuous epidermis. Complete hinge formula ,,>- - 07 > of which a large 



i\'l . x x UIOIOI. M 



part is usually deficient. 



