256 MARINE INVERTEBRATES 



crabs and Limulus, and on tnollusks. 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. crenatus. 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 EHIZOCEPHALA 



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. 



Feltogaster lives on hermit-crabs. 



