40 
b. Foot truncated or dilated at the end, often byssiferous — 
for anchoring. 
Fain. 2. Arcadee. 
Foot rather compressed, truncated or broad, and folded together 
at the base, often secreting a laminated byssus. Gills two on each 
side, subpinnate, 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. Arca. 
1. A. tetragona, t 370. f. 3. 
2. A. Nose, t. 370. f. 4. 
2. Barbatia. 
1. B. barbata, t. 370. f. 1,2, 2 a. 
2. B. Gaimardi, t. 358. f. 3. 
3. B. imbricata, t. 358. f. 4, 4 a. 
b. Pectunculina. Body roundish. Foot securiform, simple, not 
byssiferous. Cartilage in small marginal pits. 
3. Pectunculus. 
1. P. violascens, t. 371. f. 2, 3. 
2. P. pilosus, t. 353. f. 1, 2,2 a, t. 371. f. 1, 4. 
3. P. Glycimeris, t. 353. f. 4. 
4. P. intermedius, t. 353. f. 3. 
c . 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. 
4. Nucula. 
1. N. seolica, t. 355. f. 3. 
2. N. margaritacea, t. 332; f. 4, t. 372. f. 1 . 
3. N. tenuis, t. 332. f. 2. 
4. N. laterna, t. 355. f. 5. 
5. N. sulcata, t. 372. f. 6. 
Suborder II. Malleacea. 
Shell subtrigonal. Anterior adductor muscle moderate, rudi- 
mentary or none ; hinder large subcentral gill lamellar. Rectum 
over the hinder adductor muscle. Vent free, medial. Body form- 
ing a single mass. Foot byssiferous. 
Fam. 3. Pinnadce. 
Rectum with a long tubular dilatile process at the upper edge of 
