40 
B. Foot truncated or dilated at the end, often byssiferous— 
for anchoring. 
Fam. 2. Arcade. 
Foot rather compressed, truncated or broad, and folded together 
at the base, often secreting a lamimated byssus. Gills two on each 
side, subpmnate, formed of fibres. Shell subquadrate or roundish. 
Hinge-teeth two in each valve, each divided into numerous trans- 
verse interlocking plates. 
a. Arcaina. Body subquadrate. Foot byssiferous. Gills trian- 
gular, subequal, dependent, separate from each other behind. 
Cartilage in small marginal pits. 
1. Area. 
1. A. tetragona, t. 370. f. 3. 
2. A. Now, t. 370. f. 4. 
2. BARBATIA. 
Y. B: barbata, t. 370. f. 1, 2, 2a. 
2. B. Gaimardi, t. 358. f. 3. 
3. B. imbricata, t. 358. f. 4, 4a. 
b. Pectunculina. Body roundish. Foot secariform, simple, uot 
byssiferous. Cartilage in small marginal pits. 
3. PEeTUNCULUS. 
. P. violascens, t. 371. f. 2, 3. 
. P. pilosus, t. 353. f. 1,.2, 24; t. 37). f. 3,74: 
P. Glyeimeris, t. 353. f. 4. 
. P. intermedius, t. 353. f. 3. 
mo ho 
ic) 
- Nuculina. Body subtrigonal. Foot compressed, edge of basal disk 
bearded, not byssiferous. Gills small, pinnate. Lips broad, 
triangular, large. Cartilage in a small central internal pit. 
NUCUEA. 
. eolica, t. 355. f. 3. 
margaritacea, t. 332. f. 4, t. 372. f. I. 
. tenuis, t. 332. f. 2. 
. laterna, t. 355. f. 5. 
. suleata, t. 372. f. 6. 
AAAZAZzA > 
gob oto 
Suborder II]. MALLEACEA. 
Shell subtrigonal. Anterior adductor muscle moderate, rudi- 
mentary or none; hinder large subcentral gill lamellar. Rectum 
over the hmder adductor muscle. Vent free, medial. Body form- 
ing a single mass. Foot byssiferous. 
Fam. 3. Pinnade. 
Rectum with a long tubular dilatile process at the upper edge of 
