VII 



MOLLUSGA—THE SHELL 



63 



the rudimentary foot, the constitution of the mantle edge, and the absence of siphons, 

 indicate descent from sedentary forms. In the case of other forms with unequal 

 valves, however, no such descent can be established. 



In Anomia we have an example of an inequivalve bivalve, in which the valve 

 turned to the surface it rests on is iiat and the 

 upper arched. The lower valve is here the right 

 one, and takes the exact imprint of the surface 

 on which it rests, so that, for example, the mark- 

 ings of the shell of the Pecten or the Oyster, to 

 which Aiioiiiia frequently attaches itself, are 

 exactly reproduced. In this right attached valve 

 there is a perforation into which a shelly plug, 

 the calcified byssus, fits ; by means of this, the 

 animal fixes itself to its substratum. The ex- 

 planation of this perforation is seen in the course 

 of development. It commences as a simple notch 



at the edge of the shell, as found also in other ment of the shell valves of Anomia. 

 bivalves, for the passage of the byssus. By the ^' V"'*' y°™g sliell ; B, older shell with 



Fig. 62. — Three stages in the develop- 



notch for the byssus ; C, still older shell, 

 the byssus notch surrounded by the shell 



further gi'owth of the shell, this notch to a 



certain extent is grown round, and thus ap- Tnd pe7srsVing"a7aTo"le"(rfter"Morse'r 



parently travels away from the edge of the shell, 



with Avhich, however, it is still really connected (Fig. 62). In related forms {Curolia) 



this aperture becomes quite filled up by a homogeneous calcareous mass. 



Impressions on the inner surfaces of the shell. — Various organs of the Mollusc, 

 attached to or adjacent to the inner surface of the shell, leave more or less distinct 

 impressions on this surface, which are visible when it is empty. These impressions 

 are of great importance, especially to the palfcontologist, for by their means fairly 



Fig. 63. — Dimyaria, inner surface of the left shell valve. A, Cytherea cMone (SiiiupciUiafc); 

 B, Lucina Pennsylvanica (Integripalliata) ; 1, impression of the anterior ; 2, impression of the 

 posterior, adductor ; 3, sinus of the pallial line (4) ; 5, ligament. 



safe conclusions may be arrived at as to certain points in the organisation of the 

 soft body which has disappeared. 



1. The most distinct impressions are those caused by the adductor muscles. 

 Where there are two powerful adductor muscles, one anterior and the other posterior 

 (Dimyaria), there are two impressions in the corresponding parts of the inner surface 

 of the shell (Fig. 63). In cases where the anterior muscle is rudimentary, while 

 the posterior is unusually powerful, and has moved anteriorly towards the middle 

 of the shell (Monomyaria), there is only one large impression (Fig. 64). The anus 



