176 AXEL A. OLSSON 



through a round hole or foramen in the umbone of the right valve, re- 

 maining attached throughout life or only in the early stages. Valves rounded 

 or subcircular, variously inflated, but generally more or less irregular be- 

 cause of fixation, nacreous, and often with a platy or micaceous texture, 

 thin or heavy. Hinge edentulous but sometimes provided with large, diver- 

 gent resilial processes or crura simulating hinge teeth. Ligament internal, 

 attached to resilial processes along the hinge margin or to deep scars. In- 

 terior of the left valve has two or three rounded or elongated scars which 

 represent the attachment impressions of the byssal and retractor muscles. 



Because of their distorted shape, Anomias may resemble the shells of 

 oysters. They differ in their mode of attachment which is by means of a 

 byssal plug passing through an opening in the umbone of the right valve and 

 in the texture of the shell which is subnacreous, resembling thin plates of 

 mica. Surface of the valves may be smooth, folded, undulated or ribbed, but 

 an irregular development of ribs on the surface of the upper valve may be 

 merely the partial reproduction of the ribs of a Pecten or Area on which the 

 mollusk was once perched. In Anomia, the byssal foramen is a large, round, 

 open hole; in Pododesmus, the foramen is closed by a calcified plug, and 

 the surface of the lower valve is attached more closely to the substratum by 

 direct cementation. Placuanomia is attached in the early stages, later some- 

 times becoming free; the byssal foramen is sealed off by a plug. The shell 

 of Placuanomia is generally strongly plicated or folded and the hinge of 

 the right valve has two large, crural lamellae-like teeth. 



General key to genera of the Anomiidae 



I. Shell permanently fixed. 



1. Byssal foramen is a large open hole in the right or lower valve. Interior 

 of the upper valve shows three muscle impressions. 



Anomia 



2. Byssal foramen closed off by a calcified plug within the shell margin. 

 Interior of free valve with two muscle impressions. 



Pododesmus 

 II. Shell at first fixed, later sometimes becomes free. 



3. Byssal foramen closed by a calcified plug lying well within the margin. 

 Ligament attached to large, teethlike lamellae in the lower or fixed 

 valve. Surface of valves generally plicate. 



Placuanoviia 



Genus ANOMIA Linn6, 1758 

 Type species by subsequent designation, Schmidt, 1818, Anomia ephip- 

 pium Linne. 



Shell subcircular, always somewhat irregular and distorted, the right 

 valve attached to the substratum by a calcified byssus which passes through 

 a round hole in the umbone of the right or lower valve, its dorsal margm 

 usually not fully closed. The lower valve is usually sornewhat heavier 

 and often more deeply colored, the upper or free valve thinner and more 

 convex. The interior of both valves has a central thickened area; that of 

 the right valve carries a single adductor scar; that of the left valve has 

 three scars, the lower posterior scar representing the impression of the ad- 

 ductor muscle and the other two scars that of the byssal retractor muscles. 



