4 MOLLUSCA FROM THE CRAG. 



that division or section called Monomyaria, or Unimusculosa, by some authors. In this, 

 the muscle is placed in the centre, or nearly so, and is generally large and powerful, 

 adhering strongly to the interior, leaving often a deep indentation which is sometimes 

 of a different colour to the rest of the shell ; the form of this muscle mark is variable 

 in different genera, but is not of much assistance in specific determination. Some of 

 these have the hinge ligament on the exterior, like the Oyster, &c., where it acts by 

 contraction and elongation ; in others, Pecten, &c., its action is by expansion and com- 

 pression ; in this group, the edges of the mantle are generally disunited and not pro- 

 longed into siphons, and the impression formed by its muscles within the shell, is 

 without any inflection, and parallel to the margins of the valves. In the much larger 

 portion, called Dimyaria, or Bimusculosa, the animal has two distinct adductor muscles, 

 one of which is situated near the anterior margin, while the other occupies generally a 

 corresponding position on the posterior side. 



As these muscular impressions are relatively situated in the same position, and 

 always of the same form,, a great alteration takes place during the growth of the 

 animal by a gradual progression, as it increases in size and the shell enlarges ; the 

 successive advancement of these impressions is indicated in many species by distinct 

 lines of growth : and as this enlargement necessarily increases outwardly, the animal 

 possesses the power of making fresh additions to the exterior portion of the muscle, 

 while at the interior part, the now becoming useless or inconvenient portion, is 

 detached from its former place of adherence, and absorbed by the animal ; while in 

 most species, a fresh layer of calcareous matter, secreted from the whole surface of 

 the mantle, is deposited upon the interior of the shell, and covering the deserted 

 portion of the muscle mark, leaving untouched that part only against which is 

 attached its powerful adductor. In the Oyster, more especially, these successive 

 layers are distinctly visible, showing the enlargment of the shell by the extension 

 of the mantle in the lines of growth upon the exterior, as also by the generally 

 rugose or lineated surface of the ligamental area. The same may be said of the 

 dental characters of the shell which are always relatively placed in regard to the 

 specimen, whether in the young or in the adult ; and the alteration, therefore, of their 

 position in the growth of the shell, can only be effected by the removal of one part, 

 while fresh deposition is formed on the other, unless the whole be sufficiently organised 

 to partake of the varying changes of the animal itself: a question as yet not satis- 

 factorily determined. 



Dr. Carpenter gives in his ' Report on the Microscopic Structure of Shells,' as the 

 true history of the Conchiferous Acepkala, the following account : " The margin only 

 of the mantle has the power of giving origin to the outer layer of the shell, while the 

 whole surface may generate the inner. Every new production of shell consists of an 

 entire lamina of the latter substance, which lines the whole interior of the old valve, 

 and of a broader margin of the former which thickens its edge. So long as the 

 animal continues to increase in dimensions, each new exterior layer of shell projects so 



