586 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 58. 



Nematodh'us tnauritanicus and N. dromedarii differ from each 

 other chiefly in the fact that in N. mauritanicus the vulva is located 

 in the posterior body region, while in N. droniedarii it is in the an- 

 terior region. Although these forms have long spicules the other 

 characters seem to show that they are not so close to Mecistocinnis as 

 is N. neoto77ia. However, all forms with long spicules seem to be 

 closely related and the establishment of the genus Mecistocirrus to 

 contain some of them merely because in these forms the vulva is 

 located within a few millimeters of the anus instead of farther for- 

 ward as in the others does not seem to be at all justified. On the same 

 ground one would have to make a new genus for N. dromedarii^ and 

 possibly also for N . mauritanicus, for in both of these forms the bulk 

 of the reproductive organs in the female lies well back behind the 

 vulva, both ovijectors even being placed behind the vulva instead of 

 one on either side, as in the fllicoUis group and in N. iieotortm. The 

 position of the vulva seems to be the most variable character in the 

 females of this group, and can hardly be regarded as any more than 

 of specific value. The only other recorded character distinctive of 

 the genus M ecistocii^i'us is the presence of cervical papillae; but 

 minute papillae are also described for N. mauritanicus. In the va- 

 riation in the position of the vulva and in the disposition of the 

 female reproductive organs this group seems to be unique among the 

 parasitic nematodes. Seurat in 1918 divided parasitic nematodes into 

 three groups : Amphidelphs, in which the vulva is located somewhere 

 near the middle of the body and the ovijectors are placed one in front 

 and the other behind ; Opisthodelphs, in which the vulva is anterior 

 and the ovijectors are both behind; and Prodelphs, in which the 

 vulva is near the anus with the ovijectors anteriad. He attached very 

 deep phylogenetic significance to these three types. Here, however, 

 all three are found in a single group of evidently very closely re- 

 lated forms. 



The fllicoUis group contains the Species N. furcatus, N. fiUcoUis, 

 N . helvetia/rius, and N. dbnoi'malis in a series of possible relation- 

 ships ; with N. spathiger as a branch coming off somewhere between 

 N. filicoUis and N. helvetianus. 



Some of the confusion among species in the genus is apparently 

 due to the fact that Railliet's description of N. spathiger is evidently 

 based in part on females of N. dromedarii. His measurements of 

 females and eggs, consequently, were much too large for N. spathiger. 

 He also described thickenings of the eggshells at the ends which, 

 however, I have not observed in the specimens that I have examined. 



As no specimens of N. roseidus were available for study, and as the 

 published descriptions of this species are brief and unaccompanied by 

 illustrations, I am unable to suggest its relationship to other species 

 of the genus. 



