CONCHIFERA. 



293 



Order II. 



HETEROMYAKIA. 



The shells of this order have two adductor muscles, and 

 therefore two muscular impressions, but they are at very 

 unequal distances from the edge. 



Family 1. — MY TIL ACE A. 



The animal has the mantle almost entirely cloven (with 

 the exception of Dreyssena) as far as the hinder part, 

 where a small tube is formed ; the mouth is tolerably 

 large, and has two pointed tentacles which are partly 

 joined ; the foot is thin, cylindrical, and has a byssus 

 behind at the base ; the two gills are generally tolerably 

 similar ; the adductor muscles are very unequal, the fore- 

 most one is so small that it may easily be overlooked. 

 The shell varies from triangular to oval, equivalve, closed, 

 covered with a thick epidermis ; the ligament is linear, 

 almost internal. Natives of all zones, generally marine, 

 but some are found in fresh waters. 



Mytilus. Lin. — Shell longitudinal, 

 equivalve, ham-shaped, somewhat tri- 

 angular, bosses placed at the extreme 

 end, hinge mostly without true teeth ; 

 ligament linear, internal ; attached by 

 a byssus, and covered by an epidermis. 

 Animal oblong, its mantle freely open, 

 in the ventral and branchial regions; 

 ventral margin simple, branchial fur- 

 nished with pinnated fringes ; anal 



u 3 



Mytilus edulis. 



