41 
the base. Vent medial, simple. Front adductor muscle large. 
Gills narrow, very much produced behind, free from each other and 
the mantle, but fitting against a fold on its inner surface. Foot 
cylindrical, grooved beneath, large, byssiferous. 
1. Pinna. 
1. P. squamosa, t. 368, t. 369. f. 1. 
Fam. 4. Aviculadce. 
Front adductor muscle very small, rudimentary, or entirely want- 
ing. Gills narrow, much produced behind, free from each other 
and the surface of the mantle, but fitting into a fold on its inner 
surface. Rectum simple, central. Foot slender, cylindrical, grooved 
in front, generally byssiferous. Shell internally pearly ; hinge 
toothless, or nearly so. 
a. Pteriacina. Foot byssiferous. Mantle lobes fringed. Shell 
eared, with a byssal notch. 
1. Avicula. 
1. A. tarentina, t. 367. f. 3, t. 373. f. 1. 
2. A. hirundo, t. 373. f. 3. 
b. Crenatulina. Foot not byssiferous. Mantle fringed behind. 
Shell not eared, without any anterior byssal groove. Hinge 
toothless. 
1. Crenatula. 
Fam. 5. Vulsellidee. 
Anterior adductor muscle none. Gills narrow, much produced 
behind, united together and to the inner surface of the mantle, 
dividing the mantle-cavity into two parts. Rectum simple. 
1. Vulsella. 
Suborder III. Ostracea. 
Shell suborbicular. Anterior adductor muscle none ; hinder 
large, central. Cartilage internal, in a pit. Mantle often bearded 
on the whole edge. 
A. Pectenacea. Foot distinct, small, byssiferous or appendaged. 
Inner edge of mantle infexed. Tentacles separate from the 
gills. Body forming a single central mass. 
Fam. 6. Spondylidce . 
Foot short, thick, end with an enlarged truncated radiating disk, 
with a central, pedicelled, ovate body. Lips foliaceous, pinnately 
lobed. Mantle with bright ocelli. Hinge with two large interlock- 
ing teeth. Mantle edge slightly indexed 
1. Spondylus. 
1. S. gaderopus, t. 379. f. 1, 2, 3. 
