20 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Family PI NNOTH BRIDGE Milne Edwards, 1837. 
Carapace often more or less membranaceous, antero-lateral margins entire or very slightly dentate. 
Front, orbits, and eve-stalks very small. Buccal frame usually arcuate anteriorly. Outer maxillipeds 
with the fourth joint well developed, and usually the third also, the fifth articulating usually at the 
apex or at front inner angle of the fourth. Chelipeds in adult male small or moderately developed. 
Walking legs slender, with the seventh joint styliform, unarmed. The pleon of the male in general 
does not cover the whole width of sternum between the last pair of legs. 
Crabs of small size, living often in shells of bivalve mollusks, in or on the tests of Echini, and in 
tubes of worms. 
Key to the Porto Rican genera of the fam ily Pinnotheridx. 
A. Dactyli of ambulatory legs simple, acute. 
B. Third ambulatory leg very little, if any, longer than the other legs Pinnotheres 
B'. Third ambulatory leg longer and stronger than the others, usually considerably so. Pinnixa 
A'. Dactyli of first, second, and third pairs of ambulatory legs bifurcate Dissodactylus 
Genus PINNOTHERES Latreille. 
Pinnotheres Latreille, Hist. Nat. Crust., in, 25, 1802. 
Carapace smooth, subglobose, more or less membranaceous, very little wider than long. Orbits 
small, nearly circular. Front narrow, with the anterior margin nearly straight. Antennules obliquely 
transverse; merus large, usually curved, last joint of palp joined to inner margin of preceding one. 
Ambulatory legs not differing remarkably in length and of moderate length. 
Key to the Porto Rican species of the genus Pinnotheres. 
A. Dactylus of endognath cylindrical, about half as long as propodus nstrearius 
A'. Dactylus of endognath subspatulate, considerably more than half as long as propodus guerini 
Pinnotheres ostrearius, sp. nov. Oyster Crab. 
Carapace very thin and yielding, transversely oblong-orbicular, broad behind, about one-sixth 
broader than long; furrow on either side of gastric region rather deep; cardiac region less deeply 
outlined; surface smooth and shining. Front rounding downward, slightly projecting, margin truncate. 
Orbits circular, eyes partly visible in a dorsal view. Widest part of merus of maxilliped considerably 
behind distal extremity. Carpal joint oblong, and a little more than one- 
third the greatest width of merus; propodal joint about as long as carpal 
and a little narrower; last joint attached near proximal end of the inner 
margin of propodus, and very slender, cylindrical, and about one-half length 
of propodus. Chelipeds smooth; palm rapidly increasing in width from 
proximal to distal end; fingers cylindrical, half as long as palm, somewhat 
hairy, edges meeting and tips crossing when closed. First ambulatory leg 
stouter than the others; second pair the longest; first and third subeqnal;* 
the fourth pair reaches about the middle of propodus of third pair. 
Dimensions of female; Length, 7.2 mm.; width, 8.6 mm. 
Type locality, Mayaguez, in an oyster from near Cabo Rojo, 2 females, 
one of which is ovigerous (4!at. No. 23767). 
This species is very close to Pinnotheres oslreum Say, which inhabits 
the oyster of the eastern United States; it differs, however, in its softer 
integument, less orbicular outline, less swollen hands, and, above all, in the form of the outer maxil- 
liped; the last joint in P. oslreum is much smaller. 
Pinnotheres guerini Milne Edwards. 
Pinnotheres guerini Milne Edwards, Ann. Sei. Nat. (3), xx,219 (185), pi. xi, f. 9,1853. 
Pinnatercs guerini von Martens, Arch. f. Naturg., xxxvin, 105, 1872, 
Maxillipeds wide; palp very large, dactylus large and subspatulate. Hands smooth, short and 
punctate (Milne Edwards). Carapace nearly 1.5 times broader than long (von Martens). 
Porto Rieo, in oysters (Guild lach); Cuba (Milne Edwards, von Martens) . 
iws, female, (a) Chela, x 8. 
( h ) Maxilliped, x 12.8. 
