12 NEW-YORK FAUNA — CRUSTACEA. 
GENUS PINNOTHERES. Latreille. 
Form suborbicular, with the shield soft. Front broad, and covering entirely the internal an- 
tenne. Contour of the mouth semilunar: internal antenne transverse; external antenne 
short, and placed at the internal angles of the orbits. 
Oss. This genus, with four others, is included by the most recent writers in a group com- 
prising nine or ten species. They are all small, and remarkable for their singular habit of 
living within certain marine bivalve shells, chiefly of Ostrea, Mytilus, Mactra, &c. It is also 
remarkable for the singular transformations it undergoes with age. According to the obser- 
vations of Mr. Thompson (Entomological Magazine, No. 11), it appears that in the P. prsum 
of Europe, when young, the abdomen is much elongated, and ends ina fin; the shell has 
three large spines; the eyes are much enlarged; the feet dilated for swimming; in short, 
resembling very much the genus Zoe. 
PINNOTHERES OSTREUM. 
PLATE VII. FIG. 16. 
Pinnotheres ostreum. Say, Jour. Acad. Nat. Sc. Vol. 1, p. 67, pl. 4, fig. 5 (female). 
P. depressum. Ip. Ib. Vol. 1, p. 68 (male? ), Young? 
P. ostreum. Gou LD, Invertebrata of Mass. p. 328, 
Description. Femaue. Shell rounded, convex, its transverse slightly exceeding its longitu- 
dinal diameter, smooth, polished, slightly dilated behind ; its texture exceedingly membrana- 
ceous. Front not exceeding the line of the shell above. Orbits rounded or subovate; eyes 
moderate. Hands equal, smooth, with a few short hairs towards the tips, abruptly dilated 
above the origin of the thumb (see figure). Fingers with a few obsolete tubercles, and slightly 
curved at the tips. All the articulations of the feet cylindrical ; the last joints acute, with an 
impressed longitudinal line on each. Mate or Youne. Smaller; shell with a raised marginal 
line of short dense hair. Front prominent and advanced. Eyes large and prominent ; the 
last abdominal joint smaller than the preceding, and rounded : penultimate joint of all the feet 
dilated for swimming. Color, in both, reddish brown above; whitish beneath, with a dull 
yellowish transverse band. 
Length of female, 0+4; transverse diameter, 0°5. 
Length of male or young, 0°1; transverse diameter, 0°13. 
We think it extremely probable that the P. depressum of Say, is, as he himself suggests, 
the male, or as we suppose the young, of the Common Oyster Crab, as this species is com 
monly called. Mr. Say never saw but one individual, which he obtained on the coast of 
New-Jersey ; and his notes are silent as to what shell it inhabited, or whether it was in any 
shell. Some recent writers have hesitated to admit P. ostrewm as a distinct species. We 
have, however, made a direct comparison with the P. pisum of Europe, the species to which 
