1818.3 OF THE UNITED STATES. 451 



Male. Body indurated, above black, beneath yellow- 

 ish white; thorax with very short, desiduous, dense hair, 

 a dorsal whitish vitta, which is contracted in the middle 

 and near the base, cruciate before and abbreviated near 

 the clypeus, a large triangular whitish spot each side be- 

 fore the middle, and an obsolete whitish abbreviated line 

 each side behind; clypeus and anterior par t of the thorax 

 margined with whitish; /?f^/wf and abdomen with black 

 insisurcs, the latter gradually straitened to the tip, which 

 is rounded; feet, third and fourth pairs with the second 

 joints of the tibia ciliated on the posterior outer submar- 

 gin. 



Female, Body somewhat indurated, obscure brown- 

 ish, immaculate, hair shorter, more desiduous and dense 

 than that of the male; thoraji orbicular, minutely punc- 

 tured beneath the hair, a double dorsal series of irregu- 

 lar indentations, of which the hind ones are more con- 

 spicuous and sometimes confluent into an ar equated Hne; 

 feet, second pair ciliate on the inner part of the penulti- 

 mate joint, third and fourth pairs not ciliated; abdomen 

 ciliated at the edge, and obtusely carinated on the mid- 

 dle, tip entire, very obtusely rounded. 



Length, male, seven twentieths of an inch; female 

 rather more than two fifths of an inch, 



A common inmate of this species of Pinna. In one 

 shell I found the two sexes, and in another two females, 

 but this plurality rarely occurs, 



2. P. Byssomia, (Female.) Thorax somewhat trans- 

 versely oval; clypeus hardly advanced, rounded, entire; 

 hand not gibbous near the base of the thumb. 



Inhabits the Byssomia distorta. 



