MUSEUM OF COMPARATIVE ZOOLOGY. 51 



eyes on the left side anteriorly. There is no membranous capsule about the 

 brain such as is plainly seen in other forms. Mouth two thirds of the total 

 length from the anterior end. The pharynx, anterior to the mouth, is thrown 

 into deep lateral folds. Posterior to the mouth it consists of one long fold, 

 which extends backwards and occupies about one third of the pharyngeal 

 region. Sexual openings separate. The female (posterior) opening lies at a 

 distance of about one fifth the total length from the posterior end, the male 

 (anterior) opening being situated one fifth the distance from this to the mouth. 

 Owing to the specimen being immature, no traces of ova, testes, uterus, or shell- 

 gland can be detected. The penis can be dimly seen, and lying behind it and 

 opening into the female gonopore is what I take to be a bursa copulatrix. 



Eight species of Polyclads are classed by Lang (Die Polycladen, p. 629) 

 as being strictly pelagic. Von Graff Qoc. cit.), who had at his disposal a 

 greater amount of material than had before been accumulated, and who was 

 able to make sections of the difl'erent forms, reduces these to five species. Two 

 new species are added to these by him, thus making in all seven species of 

 pelagic Polyclads. Adding to these our species, Stylochoplana californica, the 

 number of pelagic Polyclads is increased to eight species. Three of these are 

 cosmopolitan, one has so far been found only in the North Pacific, and seven 

 occur in the Atlantic only. 



Prosthecerseus panamensis, sp. nov. ? 



Figures 3, 4. 



One specimen, very much mangled and torn, marked, "No. 201, Panama" 

 (a littoral form from the reef off Panama). The material was in such bad con- 

 dition that it was impossible to construct a figure of the entire animal. I could 

 not reconcile the specimen to any description of known species, but hesitate in 

 calling it a new species from the scant evidence at my disposal. The specimen 

 when intact must have measured between 30 and 40 mm. in length by about 

 20 mm. in breadth. The general color of the alcoholic material is a reddish 

 brown, the color being most concentrated along the dorsal median line, becoming 

 fainter toward the margins. The color is due to a pigment distributed in an 

 irregular meshwork, the colorless interspaces of which vary in size and shape. 

 While the pigment is densest in the median line, the colorless interspaces being 

 smaller here, at the edges it is reduced to a delicate network, thus imparting a 

 lighter color to the margins of the body. The tentacles are also pigmented, 

 and are rounded at their ends. Each tentacle has a group of eye-spots scat- 

 tered along its inner margin. The cerebral eye-spots occupy a space free from 

 pigment near the anterior end of the body. They are crowded together into a 

 common mass, which has a bilobed shape as if the eye-mass had arisen by the 

 fusion of two lateral groups. No traces of any other organs, or of the mouth or 

 sexual openings, could be identified. 



Cambridge, June 6, 1893. 



