THE LAMELLIBRANCHIA 85 



latter being distinguished by the great height and 

 thickness of the hinge-plate and large size of the teeth. 



5. Veneracea. Concentric ornament, striate or 

 costate ; occasionally radial also. Valve-margins smooth 

 or crenulate ; umbones anterior. Sinu-palliate. 



The chief genera are Mevetrix (Eoc.-Rec.), already 

 described ; Venus (Cret.-Rec.), differing from it by its 

 smaller, pointed pallial sinus, absence of lateral teeth, 

 and crenulate valve-margins; and Dosinia (Cret.-Rec.), 

 lenticular, with deep and narrow pallial sinus. 



II. Fixed Branch. 



Fixation may be temporary or permanent, by a byssus 

 or by cementation of the shell. The development of a 

 byssus tends to the abortion of the anterior part of the 

 body including the anterior adductor, to the development 

 of ears, and to an inequivalve condition. 



I. ARCACEA. 



Equivalve;isomyarian; internally porcellanous; hinge- 

 line with radial teeth, becoming secondarily taxodont; 

 cardinal area and ligament amphidetic. Ornament radial 

 or concentric. Fixation never more than temporary. 



This sub-order has generally been associated with the 

 Nuculacea as Taxodonta, but its history shows that the 

 taxodont structure of the hinge is not primary as in 

 Nucula, but is derived from a type in which the greater 

 part of the hinge is occupied by long horizontal teeth, a 

 few short oblique teeth being present at the anterior end 

 or in the centre. These characters, as well as the amphi- 

 detic cardinal area, indicate a relationship to the next 

 order, Pteriacea, from which the Arcacea diverge in 

 (a) having lost the nacreous interior, and (b) being almost 

 always equivalve. Chief genera: Area (Eoc.-Rec., 

 with doubtful records back to the Silurian), somewhat 



