ANOMIA. 329 



subovate, and almost always is broader than long, and 

 generally, too, is more or less oblique — is sometimes simple 

 at its edge, sometimes surrounded with a kind of reflected 

 margin, which is at times appressed, at times elevated (as 

 in the variety tuhularis where the contracted aperture 

 forms a very short tubular projection). This aperture, 

 which is occasionally, though rarely, suborbicular, varies 

 also in its relative proportion to the size of the valves ; the 

 average extent of its outline, as compared to that of the 

 entire shell, is about as one to three ; the severed ends of 

 the aperture lie sometimes far apart from each other, some- 

 times become almost approximate. In general, brilliancy 

 of nacre is accompanied by thinness of texture and large- 

 ness of aperture. The solid and laterally projecting strip 

 of shelly matter, which separates the perforation from the 

 edge of the upper valve is very narrow. 



When the young is uninterruptedly developed, and 

 found on smooth substances, such as the interior of dead 

 bivalves, &c., it is flat, suborbicular, and quite smooth, 

 with its beak projecting beyond the margin, and its per- 

 foration small in proportion to the dimensions of the valve ; 

 but when exposed on rough substances, it assumes quite 

 as distorted an appearance as the adult shell, to which it 

 approaches in almost every character. In the former case 

 it is the squamula of authors ; the name cylindrlca or cym- 

 liformis has been attached to such as embrace the stalks 

 of Lamellaria or other cylindraceous bodies. The mon- 

 strosity fornicata exhibits, to use the words of Turton, 

 a large vaulted chamber under the hinge of the larger 

 valve ; punctata has its upper valve embossed with tuber- 

 cular prominences, and its lower with corresponding in- 

 dentations arising from elevations present on the rock 

 coral or other marine object on which the shell may be 



VOL. II. u u 



