GENUS PINNOTHERES. 67 



tires to a secluded situation for security, but the adult fe- 

 male is protected by a male, whose shell is hard» they are 

 then called double crabs. It is sometimes infested by a 

 small worm resembling an Ascaris^ in considerable num- 

 bers; these have occurred to the number of two between 

 each of the la^ellge of the branchia. I have seen them 

 confined to the branchia of the left side only, those of the 

 opposite side were perfectly free in one instance. Worm 

 short, filiform, a little attenuated and flattened before, pale 

 yellowish, extremities roundtd, body silvery above arid 

 spotted with red. 



Genus PINNOTHERES. 



Exterior antennae short, the three basal joints longer, 

 terminal one setaceous, arising from under the interior 

 canthtis of the eye; peduncles of the eyes very short, dilated, 

 spherical. Interior antennae larger, arising from a hemis- 

 pherical base, and placed beneath the clypeus. External 

 pedi-palpi with the outer division concealed, internal divi- 

 sion with the peduncle of one subincurved joint, rounded 

 at tip and incumbent on the mentum, terminal joint bifid, 

 intermediate one subcylindrical, attached behind the ex- 

 ternal tip of the peduncle. Feet formed for walking. 



SPECIES. 



1. P, Ostreum. Thorax rounded before and on each 

 side, somewhat truncate behind, clypeus a little advanced, 

 entire; hand gibbous above near the base of the thumb. 



Inhabits the common oyster (0. Firginica.) 



Plate 4. Jig, 5. 



Thorax with minute distant hairs; punctured, punc- 



