492 



MOLLUSCA 



PHYLUM VI 



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-equcU adductors, an external parivincular liga- 

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

 spicuous epidermis. Complete hinge formula T-^r ~7j — ^i^,^', ^p of which a large 



part is usually deficient. 



Family 31. Veneridae Leach. 



Valves equal, free, closed, with prosogyrous beaks, variably sculptured, with the 

 margins more or less dentate, except in the smooth species ; adductor scars peripheral, 

 pedal distant ; pallial sinus more or less sinuated, area very distinct ; resilium usually 

 external, embraced by the ligament ; hinge-plate developed ; formula of the cardinals 



-jTT-r-rY^:^, with (t siugU obsolete lateral in one valve; the cardinals frequently bifid, 



usually radially disposed and sub-equal in size, except the posterior left one, which is often 



obsolete or obscure; supplementary cardinals or 

 rugosities are piresent in sfecialised forms. Jura 

 to Recent ; niaxiniuin in Tertiary and latei'. 



The family must be divided into at least four 

 subfamilies, as follows : — 



a. Vcnerinae : typical, with produced siphons, 

 not byssiferous, the young not retained within the 

 mother after leaving the egg. 



b. Circinac : with separate short siphons, cor- 

 relative nearly simple pallial line, sub - internal, 

 partially amphidetic resilium, and compressed beaks. 



c. Tapctinac : with long but partly separated 

 siphons, a byssus present at least in the young ; 

 hinge with no lateral teeth, otherwise like the 

 Venerinae. 



d. Gemminae : minute shells, with more or less 

 separated siphons, no byssus, obsolete lateral laminae, 

 and sheltering the nepionic young within the cavity 

 of the mother. 



Fio. 807. 



Cyprimrria discus CMnth.). Upper 

 Cretaceous ; Gosau Valley, Austria. 



Gyprimeria Conr. (Fig. 807). 



a. Pronoella Fisch. {Pronoe Ag., non Guer. 

 Men.). Compressed, pallial sinus very shallow ; 

 a posterior lateral and three cardinals in each 

 valve. Jura. 

 Like the preceding, Imt the right A^alve with only 

 two cardinals, the hinder one bifid ; pallial sinus very shallow. Cretaceous. 



Dosinia Scop. {Artemis auct.). Orbicular, lentiform, concentrically sculptured, 

 with a deep, well-marked lunule : cardinals 3:3; pallial sinus deep, ascending, 

 pointed. Cretaceous to Recent. 



Eocijclina Dall. (Cyclina Desh.). Cretaceous to Recent. Sunetta Link [Meroe 

 Schum.) ; Grateloupia Desm. Tertiary and Recent, dementia Gray. Oligocene to 

 Recent. 



Vemis Linn. (Fig. 808). Oval or rounded, plump, cordate, thick ; concentrically 

 or radially sculptured, with denticulate margins ; hinge-plate lu-oad, with three 

 cardinals in each valve and no lateral teeth ; pallial sinus short, angular. Jura to 



