ZOOL.— Vol. II.] RISEN— OLIGOCH^TA. 91 



against the supposition that UrochcBta hystrix, U. diihia and 

 Pontoscolex corethrurics are one and the same species. 



Following will be given a detailed description of Ponto- 

 scolex corethrurus subsp. mexicanus. 



External Characters. 



Size. — (figs. 1-6.) The average length of the Tepic 

 specimens is much less than that of the specimens from the 

 Cape Region. The largest of them average iii mm., the 

 smallest 95 mm.; the width in front of the clitellum at the 

 narrowest part is about 3 mm. Specimens from the Cape 

 Region measure 200 mm. in length, with a diameter of 4 

 mm. The number of somites in the Tepic specimens 

 ranges between 145 and 212. Figure i represents a spec- 

 imen natural size, narcotized and preserved in alcohol. 

 •The specimen is not, however, as extended as when crawl- 

 ing about during life. 



SelcB. — The anterior set« are strictly paired and the 

 dorsal set£e are likewise paired till they reach somite XII, 

 where they begin to separate a little, continuing about 

 equidistantly in XII, XIII and XIV, and running only 

 slightly deltoid. In XV they separate more, and in XVIII 

 they are at the greatest distance apart, continuing to remain 

 the same to about somite XLII. In or near this latter 

 somite setse c continue in a straight line to the caudal zone, 

 or for about 44 somites from the tail-end. From the caudal 

 zone to the tail-end the arrangement of the set£e is quin- 

 cuncial, in the same manner as has so often been described 

 and figured by various investigators. The difference in the 

 size and location of the setae in somite II (figs. 3 and 4) has 

 already been mentioned. 



There is a difference between the penial setse of my 

 specimens (fig. 7) and those figured by Perrier. His fig- 

 ure represents a penial seta with sixteen cup-shaped depres- 

 sions, while the setae of my specimens have only eight or nine. 



Anter'tor Somites. — (figs. 2, 3, 4.) Although Horst (17) 

 has shown that there exists a real prostomium extending 



