OSTREACEA. 



697 



of the shell — and the ligamental area is oblique and narrow. The 

 species of Exogyra are abundant in the Jurassic and Cretaceous 

 deposits. 



The name of Pernostrea has been proposed by Munier-Chalmas 

 for certain Jurassic Oysters which differ from Ostrea proper chiefly 

 in having the ligament contained in from four to eight transverse 

 grooves or pits (as in Pei'iia). 



Family 2. Anomiid^. — In this family the shell is inequivalve, 

 and is fixed to foreign objects, either in early life or permanently, 

 by a byssus, which traverses a notch or foramen in the right valve, 

 and is ordinarily calcified at its termination. The muscles of the 

 byssus leave one or more impressions on the interior of the left 

 valve. There is an internal ligament ; the hinge is edentulous ; 

 the pallial line is simple ; and the shell exhibits a laminated struc- 

 ture. The range of the family is from the Devonian to the present 

 day, and the chief genera are Anomia and Placuna. 



In the genus Ajio?nia (fig. 562) the shell is suborbicular and 

 ostreiform, the left or upper valve being convex, while the right or 

 under valve is flat or concave. In the margin of the right valve, near 



Fig. 562. — Anomia Casanovei, from the Eocene Tertiary, enlarged, a, Interior of theleft 

 valve ; b, Interior of the right valve : n, Foramen for the passage of the byssus ; m, Impressions 

 left by the muscles of the byssus ; vi , Adductor impression. (After Hoernes.) 



the hinge, is a deep notch or sinus, which in process of growth may 

 be converted into a complete foramen, and which serves to transmit 

 the byssus, by means of which the shell is attached to foreign bodies. 

 The end of the byssus is calcified, and it forms a solid plug which 

 fills up the notch or foramen in the valve. The interior of the right 

 valve shows only the impression of the single adductor, but the in- 

 terior of the left valve exhibits in addition one to three scars produced 

 by the muscles of the byssus. All the living forms of Anomia are 

 marine, and the genus is represented in past time by numerous 



