134 PROCEEDINGS OF THE NATIONAL MUSEUM vol. i04 



into the posterodorsal angle of the male antrum. No penis papilla 

 could be discerned, but the fact that the sections were not exactly- 

 sagittal made interpretation here difficult. There is a broad but 

 dorsoventrally flattened male antrum that narrows to a tubular portion 

 opening by the male gonopore, so that the entire male antrum is 

 shaped something like a mushroom. That this is antrum is shown by 

 the identity of its epithelial lining with that of the general body 

 epithelium and the marked difference between this lining and the lining 

 of the ejaculatory duct through the muscular mass. This distinction 

 does not show very well in the drawing. The muscular mass fills the 

 whole space between the dorsal antral wall and the ejaculatory duct 

 and also extends around the duct for some distance dorsal and anterior 

 to the latter. 



The female pore lies well behind the male pore and has a typical 

 leptoplanid structure. The gonopore leads into a vagina with 

 markedly scalloped walls and a tall epithelial lining surrounded by a 

 fair musculature. The vagina slopes forward, then makes a pro- 

 nounced backward curve, receives the oviducts into its ventral wall, 

 and continues as a short duct of Lang's vesicle that soon terminates in 

 a small oval Lang's vesicle. The course of the uteri in relation to the 

 pharynx could not be followed. 



Distribution: Taken by R. G. Fennah for the U. S. National 

 Museum in rock pools at Marigot, Dominica, British West Indies, 

 December 1939. 



Holotype: Anterior half as whole mount, copulatory region as 

 sagittal serial sections (three slides) deposited in the U. S. National 

 Museum, No. 24622. 



Remarks: It appeared impossible to fit this species into any of the 

 existing genera of the Leptoplanid ae. Whereas the general appear- 

 ance of the specimen and the form of the female apparatus are typically 

 leptoplanid, the curious massivity of the male apparatus and the 

 absence of a penis papilla and a formed prostatic vesicle depart from 

 the norm of the family. The relation of this genus to other leptoplanid 

 genera is not clear. 



Family Planoceridae Lang, 1884 

 Genus Gnesioceros Diesing, 1861 



Definition: Planoceridae with cuneate pellucid bodies; with 

 tentacles containing eyes; with true seminal vesicle and interpolated 

 prostatic vesicle; distal end of cirrus sac shaped lil^e a conch or cowry 

 shell, armed with parallel toothed bands; vagina with a powerful 

 musculoglandular fold; Lang's vesicle transverse. 



Type species: Plajiaria sargassicola Mertens, 1833. 



