22 
one on each side the upper part against the side of the body. 
Mantle with a small valvular hole, under the siphon. Foot with a 
byssal groove. Shell inequivalve. Umbo with a cartilage-slit. 
Cartilage internal, in a pit in each valve, with a peculiar shelly plate 
before it, or covering it. Periostraca often hispid. 
a. Siphons united. Valves subequal. Clavicle linear. 
1. Laternula. 
1. L. hispidula, t. 327, and fig. 6. 
Fig. 6 . — Laternula hispidula. 
a a, adductor muscles. I, ligament, o, clavicle, y, gills, h, palpi. 
b. Siphons separate at the end. Valves unequal. Clavicle fiat , 
covering the cartilage. 
2. Lyonsia. 
1. L. coruscans, t. 32/. f. 3, 
2. L. norvegica, t. 32 7. f. 1. 
3. Mytilimera. 
1. M. patagonica, t. 327. f. 2. 
c. Siphons separate. Valves unequal. Clavicle small, subcylin- 
drical . 
4. Thracia. 
1. T. phaseolina, t. 327. f. 5. 
2. T. distorta, t. 327. f. 4. 
3. T. corbuloides, t. 327. f. 7* 
5. Periploma. 
1. P. ovale, t. 327- f. 6, t. 328. f. 3. 
6. Ne^ra. 
1. N. costulata, t. 350. f. 2. 
2. N. cuspidata, t. 350. f. 3. 
7- Poromya. 
1. P. granulata, t. 346. f. 3. 
