ANOMIA. Zinn., 1767.) 
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 unsynimetrical, 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 are 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 
plug is found in the fossil state in the upper, though Ihave 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 1 imagined to 
have been the internal shell of a Gasteropod, and figured in the ‘Crag Mollusca,’ 
doubtingly, under the name of Ap/ysia. 
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 synonyma see ‘Craz Mollusca,’ Part ‘ Bivalves.” 
