PLOTOLEPIS NANS. 41 



Genotype. — P. nans, sp. nov. 



This genus is characterized especially by the remarkable modification of a 

 definite number of the notocirri into the elongate and more or less inflated organs 

 above described and also by the vesicular character and greatly reduced size 

 of the anterior elytra. These modifications clearly suggest adaptations to the 

 pelagic habit which is relatively uncommon in the family. Moore's Drieschia 

 pellucida (Proc. Acad. nat. sci. Phila., 1903, p. 794, pi. 50, f. 1-12), taken in 

 the Atlantic off Woods Hole, is clearly also a member of this genus, presenting 

 the same inequaUty in the notocirri, the vesicular elytra, and similar form and 

 general proportions of the setae. 



Plotolepis nans, sp. nov.^ 

 Plate 7, fig. 3, 4. 



The body and its appendages in general are colorless or but sHghtly tinged 

 with brown, apparently from preservation, and are transparent or translucent. 

 The eyes are black and the setae are colorless and transparent. 



The body is very short. It is widest in the anterior half where the diame- 

 ter does not vary much ; but from near the middle caudad it narrows uniformly 

 and conspicuously. The tj^e is but 6.4 mm. in length. The greatest width 

 over all, that is from tip to tip of opposite setae, is 7 mm., thus exceeding the 

 length ; and the mdth across the body alone is but 1 mm. The total of somites 

 in the type is only twenty-one. The number of pairs of elytra is eleven. 



The prostomium is about three fourths as long as wide. On each side it 

 is evenly convex or weakly angular at the middle. Anteriorly it presents an 

 excision in which the cirratophore of the median tentacle fits. On each side 

 of this the horn of the prostomium bears the lateral tentacle. It is convex 

 above and has the usual mecUan longitudinal furrow, wliich is narrow and weak 

 mesally but mdens anteriorly where it passes into the anterior incision and 

 posteriorly where a narrow triangular tongue extends into it from the second 

 somite. The four eyes are small as usual, with the anterior a httle the larger. 

 The anterior eyes are much more widely separated than the posterior, and are 

 situated on the sides of the head, the posteriors being more dorsal and each 

 facing ectocaudad. The two eyes on each side are separated by less than their 



' nans, a swimmer. 



