36 INTRODUCTION. 



the first is named " Dimyaria/' having two adductor muscles ; 

 and the second, '^ Monomyaria/^ having but one. These 

 adductor muscles are used for the purpose of drawing the 

 valves together, being composed of contractile fibrous gristle, 

 fastened firmly to the inner surface of each valve. The place 

 where they are thus fixed may be seen when the animal is 

 removed by depressed areas, which are generally pretty well 

 defined and are named muscular impressions. Where there 

 is but one adductor muscle, there will be but one of these im- 

 pressions near the centre of each valve, but in the Dimyaria, 

 where there are two, the impressions are seen, one on the 

 anterior, and one on the posterior of each valve, just below 

 the hinge lamina. They are sometimes complex, that is, com- 

 posed of several portions in a group ; but in general, they are 

 simple and well defined. 



They are also described as large or small, in proportion 

 to the size of the shell ; regular or irregular in form. The 

 animal is attached to the inner surface by the fibrous 

 portions of the mantle, which creates a hnear impression 

 or cicatrix, commonly described as the palleal impression, or 

 muscular impression of the mantle. It runs near the ven- 

 tral margin from one muscular impression to the other, some- 

 times in a smooth continuous line or band, and sometimes in 

 an interrupted series of small impressions. Near the point of 

 union with the posterior muscular impression, there is some- 

 times a more or less considerable winding inwards towards the 

 centre of the shell, and back again towards the point of union. 

 This is named the sinus, and is distinguished as being angular 

 or rounded, large or small, according to the species. When 

 it enters towards the centre of the shell in a tongue-shaped 

 outline it is said to be ligulate. Where it exists it aff'ords a 

 certain index to the posterior side of the shell ; as it is the 

 region through which the excretory tubes pass. 



