2 CONCHIFERA. 



leaf-like mantle. The respiratory organs, though with- 

 in the mantle, are free from it. The two valves of the 

 shell, which protect the animal, are moulded on, and 

 secreted by, the mantle. These valves are united at the 

 back by a ligament, which causes them to expand, and 

 they are drawn together by an adductor muscle attached 

 to the inner surface of each shell. So simple is the 

 construction of the animals that the dissection of a few 

 individuals will convey more knowledge than a volume 

 of explanation. 



In this section, only three tribes inhabit the fresh 

 waters of this country. 



Tribe I. — CvcLADiDiE. 



Animal — With front part of the mantle open and plain at the 

 edge ; tube of the siphon produced, and either partially 

 separated from their unfringed extremities, or completely 

 united to them. Foot large, and tongue-shaped. 



Shell — Thin, rather swelling, nearly equal, striated and fur- 

 rowed, covered hy an epidermis, ligament outside, hinge 

 with lateral and cardinal teeth. All the British species 

 are ovoviviparous. This tribe is divided into two genera 

 — Cyclas and Pisidium. 



Tribe II. — Unionid^. 



Animal — With margins of mantle incapable of being refiexed, 



freely open almost the entire length ; a difference in the 



structure of their hinder borders alone indicates thesiphon- 



tubes ; short cirrhi in the branchial region ; foot large, 



broad, compressed. 



Shell — JEquivalve: covered with an epidermis ; hinge with or 

 without teeth ; ligament external. It includes two genera 

 — Unio and Anodonta. 



Tribe III. — Mytilid^. 



Animal — With edges of mantle united all round except three 

 apertures, one below for the passage of a short strap- 

 shaped foot, with a byssus at the base, and a distinct 

 byssal groove, and two behind ; the upper one a hole only, 

 and the lower one produced into a siphon ; mouth large 

 and refiexed. 



