256 MARINE INVERTEBRATES 
crabs and Limulus, and on mollusks. It is chiefly found on objects below 
low-water mark. It ranges from Massachusetts Bay to Florida. It is 
easily distinguished from the preceding by its low, broad form and 
shelly base. The shell is smooth and circular at the base, and inclines 
backward, forming an oblique cone with a triangular opening ; the plates 
terminate in points at the summit and incline backward, the last one 
forming a kind of beak. The operculum is pyramidal ; two of its valves 
have both transverse and longitudinal lines well defined at the base, and 
are coarsely toothed at the edges; the posterior valves are slightly 
grooved across. 
B. rugosus (Gould), B. erenatus. Shell white, cylindrical, some- 
what conical, rugged, the summit usually as broad as the base; height 
often greater than diameter; aperture diamond-shaped, plates ending at 
the summit in acute spreading points, the posterior plate folded and 
curved like a beak; plates rough, with coarse, irregular ribs; valves of 
the operculum at the summit acute, with diverging points; the points 
striated. Found on shells and stones in deep water and also on bottoms 
of ships. 
B. geniculatus. Shell dirty greenish-white, cone-shaped; aperture 
about the size of base; shell-plates triangular, unequal in breadth, and 
with alternate large and small ribs; the smaller ribs compressed and 
roughened on the edges by the conspicuous lines of growth which run 
across them ; depressed areas between the plates marked with fine cross- 
lines; front valves of the operculum have coarse plated ridges, which 
incline over one another and are crossed by fine radiating lines; diam- 
eter at base one to one and a half inches ; height two thirds the diameter. 
Found on pecten shells, and abundant off the coast of Maine. 
B. tintinnabulum. Shell pink to purplish, conical, with six triangular 
plates, which are grooved, forming unequal ribs, and crossed by distinct 
lines of growth ; spaces between plates crossed by lateral lines; posterior 
valves of the operculum longer than the others and curved forward, 
resembling the beak of a bird of prey; diameter at the base one inch; 
height one and a half inches. Found in warm waters and on vessels 
from the South. 
SUBORDER RHIZOCEPHALA 
These are parasitic forms and very degenerate. Sacculina lives 
on crabs, and its term of life is about three years, during which 
period the afflicted crab does not moult. Its shape is that of an 
ovoid sac on a stalk, which it attaches between two segments of 
the ventral surface of its host. The stalk divides and ramifies in 
a root-like manner within the body of the crab, from whose vital 
elements it absorbs its nourishment. The roots spread like a 
mycelium through the whole crab, even to the claws. 
Peltogaster lives on hermit-crabs. 
