294) POPULAR BRITISH CONCHOLOGY. 



muscle, which has to be separated from the attachment. 

 It forms the gristle, a portion of which often adheres to the 

 shell after the rest of the animal has been removed. Bivalve 

 mollusca have been divided into two orders, namely, Bi- 

 myaria, having two of these adductor muscles, and, conse- 

 quently, two impressions in each valve ; and Mo7iomyana, 

 having only one. The Venus tribe present familiar ex- 

 amples of the former; while the Oysters and Pectens are 

 specimens of the latter. In the Limyaria the two impres- 

 sions are seen opposite each other, near the anterior and 

 posterior ends of the shell ; and there is generally to be 

 traced a line or impression leading from one to the other. 

 This is termed the pallial impression, or impression of the 

 mantle. In some cases this impression is entire, leading 

 in one uninterrupted carve from end to end, parallel with 

 the ventral margin ; but in others it has a sinus, or winding, 

 at the posterior end, before it joins the posterior muscular 

 impression. The existence, depth, and shape of this sinus 

 is deemed an important character in determining the bounds 

 of genera and species. 



Many words remain, the. applications of which are too 

 obvious to need special ex^^lanation ; and others, which 



