CONCHIFEEA. 
249 
Order IV. Unioxacea. — 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. Unionidce. (p. 270.) 
Sub-order III. Mytilacea. — 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 edge. 
