25 
3. L. pygmaea, t. 372. f. 3. 
4. L. emarginata, t. 332. f. 1, t. 372. f. 7- 
** Cartilage external, marginal. 
3. Ctenoconcha. 
1. C. australis, t. 372. f. 10. 
Fam. 21. Modiolarcadee. 
Gills four, thick, dependent, subtrigonal, truncated in front, nar- 
row, produced, and united together behind the foot. Lips four, 
moderate. Palpi obsolete. Siphonal apertures distinct; anal 
moderate ; inhalent very large, inferior, simple-edged. Foot ob- 
long, truncate, lanceolate, acute in front, with subposterior central 
hole for byssus. Eggs retained in outer gill-cases. Shell equivalve. 
Hinge-teeth none or rudimentary. Cartilage external, linear. Peri • 
ostraca polished. Living attached to floating sea-weed. 
1. Modiolarca. 
1. M. trapezina (tig. 8). 
Fig. 8 . — Modiolarca trapezina. 
p p', pedal muscles, a a', adductor muscles. /, foot. I, liver. 
V. Foot elongate, slender, strap-like, byssiferous—for anchoring 
the animal. 
Fam. 22. Dreissenidce. 
Foot conical, small. Anterior adductor muscle small, on a trans- 
verse subumbonal internal plate. Mantle edge double. Gills four, 
unequal ; outer small. Lips four, triangular, short, broad at the 
base. Shell equivalve, subtrigonal, keeled. Hinge toothless. Car- 
tilage external, marginal. Fluviatile. 
1. Dreissena. 
1. D. polvmorpha, t. 333. f. 1, 2, 3, 5. 
Fam. 23. Galeommidm. 
Foot very small, strap-shaped, flattened beneath. Mantle edge 
