MOLLUSCA. 25 



where the muscles and mantle or pallium are attached ; and are 

 called the muscular and palleal impressions. The latter usually 

 turns inward towards the centre of the shell, and then outwards, 

 forming a notch or sinus, more or less deep, at one end of the 

 shell ; this is always the hinder end. 



There is sometimes only one muscular impression, but gen- 

 erally there are two, in each valve. These serve as permanent 

 marks by which to divide the conchs into two groups, the Uni- 

 musculosa and Bimusculosa. 



The following synopsis may enable one to form a near approxi- 

 mation to the genus of any of our bivalve shells. It has been 

 drawn up without any reference to a natural arrangement, and 

 merely to aid the student, by grouping the shells in an artificial 

 manner, to come more easily at their true place and name. 



I. Shell transverse or rounded, with tioo muscular impressions. BIMUSCULOSA. 

 i. Shell gaping. 



1. gaping at base and ends, hinge toothless. 



* united by additional valves, Pholas. 



** united by ligament, Panopc^a, Glycy'meris, Ter&do. 



2. gaping at the ends only, and with hinge (cardinal) teeth. 



* Cardinal teeth simple, Solen, Solecurlus, Solemy^a, Machce^ra. 

 ** Cardinal teeth spoon-shaped. 



t without lateral teeth, Mya, Anattna, Montacuta, Cochloddsma, 



Osteodesma. 



tt with lateral teeth, Mactra, Mesodesma, Cumingia, K6llia. 

 ii. Shell closed. 



1. inequivalve, Corbula, Pandora, Thracia. 



2. equivalve. 



* inhabiting the sea. 



t teeth rounded, ascending, Saxicava, Petricola. 

 it teeth compressed. 



sub-parallel, Psammdbia, TeWina, Lucina. 

 diverging. 



a. without lateral teeth, Cypnna, Cyther&a, Venus, 



Astdrte. 



b. with lateral teeth, Cdrdium, Cardita. 



** inhabiting fresh water, U^nio, Masmodon, Jl'nodon, Cyclas. 



II. Shell fan-shaped or irregular, with one muscular impression. UNIMUSCULOSA. 

 i. united by an external ligament, Modiola, My'tilus. 



ii. united by an internal cartilage. 



1. shell regular, Pecten. 



2. shell irregular. 



* entire, O'strea. 



** valve perforated, Anbmia, Terebrdtula. 

 4 



