PACIFIC COAST OLIGOCn.ETA. 73 



PONTODRILUS Grube. 



• The genus Pontodrilus, together with Pliotodrihis, is distinguishetl from all ro- 



lati'd genera l)v the absence of nephridia in the twelve anterior somites. In the 

 majority of sj)ecies the nephridia commence in somite xiii, but in two species they 

 commence respectively in xivand xv. A common character to all the species appears 

 to be the very great thickening of the septa between somite v and xiii. 



I have so far on the Pacific Coast only found one species of the genus Ponto- 

 drilus. Examinations were made from alcoholic specimens, I having no oj)portunity 

 to examine them when T found them in their native habitat — Mexico. 



Pontodrilus Michaelseni n. sp. 



This .species differs from all other species, except P. Maronis* which have been 

 referred to this genus in possessing a glandular crop occupying somites xiv, xv and 

 xvi, as well as in other minor details. The habitat of the species is the very narrow 

 moist line between high tide and dry soil on the shores of the Gulf of California 

 around Guaymas, Mexico. The soil in which it occurs is very sandy and thoroughly 

 soaked or moistened with the strongly saline water of the gulf. It occurred here in 

 large numbers, but unfortunately at ray visit most of the specimens were immature, 

 only two possessing clitellum in the end of November, 18D3. I dedicate this species 

 to Dr. W. Michaelsen of Hamburg, wiiose labors in our common field are among the 

 the most thorough and best. 



EXTERIOR CHARACTERS. 



The body of this species reaches a length of oj inches with a width at the 

 clitellum of less than ^ of an inch, but the raajoi'ity of specimens are somewhat 

 smaller. The above measurement refers to specimens slowly killed and then hard- 

 ened in alcohol. The body is tapering towards the tail end, the latter however being 

 slightly swollen (fig. 2-4). 



The prostomium encroaches on somite i, dividing it about one-half. Somite i 

 is slightly larger than any of the following somites (fig. 25). 



The clitellum commences in xiii and in full-grown specimens includes part of 

 xix. It is incomplete in a peculiar manner. In xiii to xvii inclusive it is only 

 developed on the dorsal side of the body. But in xviii and in pai*t of xix the clitellum 

 is only developed on the ventral side of the body, though this fact cannot be ascer- 

 tained from exterior inspection. Viewed from the underside the clitellum appears to 

 be on a line drawn through seta one. Between xvii and xviii the clitellar swelling 

 recedes slightly, again to widen out in xviii, and here joining to a pair of ventral 

 cushions, between which and the clitellum proper are situated the male pores. 



Another pair of swellings are noticed around the spermathecal pores, covering 

 on either side parts of somites vi, vii and viii (fig. 26). 



*I received M. Perrier's memoir on Pontodrilus only after this jjaper was parlly in print. 



