Lamarck's Genera of Shellx. 29 



it were, in the base of the valves. Hinge without teeth, %vavy, 

 subsinuous, unequaf. Two distant, lateral, oblong muscular 

 impressions. Ligament external, tortuous, partly penetrating 

 the shell. 



The etheriaj are very rare shells, and little known, being 

 attached to rocks at a considerable depth in the sea. They 

 might be mistaken for ostrese, from their irregular form, but 

 they are allied to the chamse by their separate, lateral, bi- 

 rauscular impressions, and indeed are only distinguished from 

 them, by having no tooth at the hinge ; they are, however, much 

 more pearly and brilliant than the chamee internally, and their 

 shell is perfectly foliated, like that of the ostve*. Mostof thera 

 are rather large, and all are attached by the lower valve. 



This genus is subdivided into (I) shells having an oblong 

 callus in the base of the shell, 2 species ; and (2) those which 

 have no such callus. 



Type. Etheria semilunata *. 



Shell obliquely ovate, semi-circular, rather gibbous ; posterior 

 side straight ; nates conformable, nearly equal. Indiaii Oce(m } 

 In all 4 species. PI. L Fig. 75. 



Second Order. 



CONCHIFERA UNIMUSCULOSAf. 



Only one muscle, which appears to pass through the body. 

 Shell with one internal muscular impression, nearly in the centre. 



The distinguishing characteristic of this order is the singular 

 muscle by which the animal is attached to its shell, the impres- 

 sion of which, is generally discernible in each valve, sometimes 

 very large and remarkable. The shell is generally irregular, 

 inequivalve, and of a foliated texture ; but, besides that these 

 characters are not peculiar to the genera belonging to this order, 



• Cresrent-shaped. Lamarck's first species of the second subdivision. 

 The shell, from which our figure is taken, was obligingly lent us by Mr. 

 Sowerby. It is extremely difficult to determine the species of some of the 

 irregular shells, whose forms are liable to almost infinite variations. We 

 think our specimen is pretty certainly E. semilunata, but possibly that, and 

 the other, non-callous shell, E. transversa, given by Lamarck, may be 

 merely varieties in shape of the same species, 

 t Having one tniiscU: 



