328 OSTREADiE. 



and by far the greater portion, are the result of accidental 

 distortion, or the mere modifications of form, or that suppo- 

 sitious sculpture which all members of a sessile genus are 

 liable to. 



In the most freely-developed specimens — and these are 

 less frequently met with in the adult than in the immature 

 state — (the chances of the growth having been uninter- 

 rupted materially diminishing with age), the shape is sub- 

 orbicular, though rather longer than broad, and the sides 

 are nearly equal. The texture is pearly, and in the mass of 

 examples is thin, white, a little transparent, and destitute 

 of any natural sculpture whatsoever. In the large coarse 

 specimens, however, to which the name epMjypmm is usually 

 restricted by collectors, the texture is moderately thick, and 

 at times almost opaque ; the shape is more produced, the 

 sides decidedly unequal, and the surface, from the shell 

 being ordinarily attached to oysters, correspondingly, though 

 in a diminished ratio, sublamellar, or concentrically girt 

 with undulating wrinkles. Such as are found on Pectens 

 imitate their radiating ribs ; yet it is not generally diflScult 

 to determine, even when an Anomia is detached, whether 

 the apparent costse are natural to the species, or of ex- 

 traneous origin, by observing whether all the ribs emanate 

 from the umbo, whence they invariably radiate in all 

 essentially ribbed species of this genus. The beaks are 

 acute and terminal, but bend either to the right or left : 

 occasionally, also, they are subcentral and inflected. The 

 upper valve is usually the more ventricose and solid ; the 

 lower, the thinner and more flattened ; occasionally, how- 

 ever, the latter — which, except at the beaks, is almost of 

 equal dimensions to the former — becomes, from the neces- 

 sities of its position, wholly or partially convex. The 

 perforation — which in the more characteristic examples is 



I 



