ANOMIA. Linn., 1767. 1 



Generic Character. Shell inequivalved, irregular and variable, subequilateral, ovate 

 or suborbicular, and slightly pearly within ; upper or left valve convex, smooth, lamellar, 

 striated, costated or muricated ; lower valve flattened, sometimes very slender, with 

 a large foramen, through which passes a calcareous appendage, or calcified byssus, for the 

 attachment of the animal. One muscular impression in the lower or fixed valve, and 

 four in the upper. Connexus cartilaginous ; hinge edentulous. 



Animal unsymmetrical, with the edges of the mantle disconnected, except at a small 

 spot near the hinge ; its margin double, slightly fringed, without ocelli or rudimentary eyes ; 

 foot very small, cylindrical, expanded at the end, and grooved ; byssus large, passing through 

 a nearly complete foramen in the right mantle lobe, and attached by a powerful muscle to 

 the centre of the left valve. One adductor muscle ; palleal line continuous. Sexes distinct. 



The impressions of four muscles arc left upon the interior of the upper or left valve ; 

 one of the four is that of the adductor, and is the only one impressed upon both valves. 

 The largest of these muscle-marks is the attachment of the byssal plug ; probably the two 

 centre marks belong to that organ, and the small one in front of the cartilage-pit is caused 

 by the retractor of the foot. The animal cements itself to the rock by the byssus, which 

 contains so much calcareous matter that it becomes as hard as the shell itself, and this 

 ping is found in the fossil state in the upper, though I have not yet seen it from the lower, 

 Tertiaries. 



The right or adherent valve is very thin, oftentimes almost obsolete, and in some 

 species it is much less in size than the upper, so that the mantle extends considerably 

 beyond the edge of the shell, showing the lower valve to be almost useless ; the perforation 

 of this valve in some species is very large, with an unconnected margin ; indeed, this is the 

 more common character. The cartilaginous connexus is placed on a projecting piece of 

 this valve, behind which the shell is thickened with a sort of double ridge running into 

 the body of the valve ; this is often the only portion preserved in the fossil state, and I 

 now find that to have been the condition of the little Crag fossil, which I imagined to 

 have been the internal shell of a Gasteropotl, and figured in the ' Crag Mollusca,' 

 doubtingly, under the name of Aplysia. 



The umbo of the upper valve in some specimens of this genus is removed to a con- 

 siderable distance from the margin of the shell, and in its exterior makes, in appearance, 

 an approach to the limpets. The large and extended muscle-marks of the interior 



' For generic syuonyma see 'Crag Mollusca,' Part "Bivalves." 



