ZooL.— Vol. II.] RISEN— OLIGOCHMTA. 215 



The single specimen from the City of Mexico differs in 

 some interesting particulars from the specimens from 

 Toluca. The ventral part of somite VIII possesses a genital 

 zone consisting of a small rectangular depression surrounded 

 by two parallel ridges which are flanked by two small 

 papillae , the whole structure occupying the space of the 

 ventral rows of setse. Sections of this specimen show a 

 pair of prostates situated with the sperm-ducts in somite 

 XVIII. These prostates, which have the same size and 

 shape as those in XVII and XIX, open into the posterior 

 part of the somite, between the equator and the interseg- 

 mental groove. The sperm-ducts seem to fuse in somite 

 XVI, opening as is usual in this species into the interseg- 

 mental groove between XVII and XVIII. Adjoining these 

 prostates in XVIII are small penial set^ of the same struc- 

 ture as those opening with the regular prostates in XVII 

 and XIX. 



It is interesting to note that an exactly similar prolifera- 

 tion of prostates in the somite of the spermiducal pores was 

 found in a specimen of Trigaster tolteca previously described 

 in this paper; it is also found constant in some species of 

 Dichogaster , 



Detailed Description. 



The limited number of specimens and their indifferent 

 preservation makes it impossible to enter as fully into details 

 as could be wished. This refers principally to points of 

 histological interest. 



The -prostomium divides somite I completely, and a deep 

 crease is projected as far backwards as the anterior half of 

 III. 



The somites are of even size and smooth. Those con- 

 taining the spermathecge are, however, furnished with a 

 deep crease on the ventral side on the posterior half of the 

 somite. The genital zone in the most perfect of the speci- 

 mens consists of a rectangular depression bounded interiorly 

 by the two more or less parallel grooves. Immediately 



(9) • December 23, 1899. 



