12 NEW-YORK FAUNA — CRUSTACEA. 



GENUS PINNOTHERES. Latreille. 



Form suborbicular, with the shield soft. Front broad, and covering entirely the internal an- 

 tennas. Contour of the mouth semilunar : internal antennas transverse ; external antennae 

 short, and placed at the internal angles of the orbits. 



Obs. 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, <kc. 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.pisum 

 of Europe, when young, the abdomen is much elongated, and ends in a 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 Vn. FIG. 16. 



Pmnotheres ostreum. Say, Jour. Acad. Nat. Sc. Vol. 1, p. 67, pi. 4, fig. 5 {female). 

 P. depressum. Id. lb. Vol. 1, p. GS (male ? ). Young ? 



P. ostreum, Gould, Invertebrata of Mass. p. 328, 



Description. Female. 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 lips. All the articulations of the feet cylindrical ; the last joints acute, with an 

 impressed longitudinal line on each. Male or Young. 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 jsreceding, and rounded : penultimate joint of all the feet 

 dilated for swimming, Colo?; 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. ostreum as a distinct species. We 

 have, however, made a direct comparison with the P. pisum of Europe, the species to which 



