CONCHIPEEA. 249 



Order IV. Unionacea. — Mantle lobes free, 

 slightly united behind, forming a separate anal 

 siphonal opening placed under the hinder ad- 

 ductor muscle. 

 ■ Sub-order I. Lucinacea. — Foot cylindrical, 

 elongate, inferior ; anterior adductor muscle 

 generally elongate. 

 Sub-order II. Submytilacea. — Foot large, com- 

 pressed ; anterior adductor muscle nearly as 

 large as the hinder ; shell free ; a periostraca 

 hard and small. Fluviatile. 

 4. UnionidcB. (p. 270.) 

 Sub-order III. My tilacea. — Foot small, ligulate, 

 byssiferous ; anterior adductor muscle small. 



Order V. Pectinacea. — Mantle leaves free all 

 round, without any separate opening for the 

 out-going current and vent. 



Sub-order I. Arcacea. — Shell oblong or roundish; 

 the anterior and posterior adductor muscles 

 subequal ; mantle bearded behind ; hinge 

 teeth deeply grooved or divided into trans- 

 verse interlocking plates. 



Sub-order II. Malleacea. — Shell subdiagonal; 

 anterior adductor muscle small, rudimentary ; 

 hinder large, subcentral ; gills laminal. 



Sub-order III. Ostracea. — Shell suborbicular ; 

 anterior adductor muscle obliterated; hin- 

 der large, central ; cartilage internal ; mantle 

 often bearded on the whole edgre. 



