621 



Described from seventeen sexually mature specimens. Type and 

 paratypes in the collection of the junior author; paratypes in the 

 collection of the senior author. 



The specimens which fonn the basis of this description were 

 found in a compost pile in the forest grounds of the University of 

 Illinois and were collected in October and November, 19 lo. 



AfRnities. — This species seems to approach several other species, 

 namely, F. lohifera Vejdovsky ('79, p. 57), F. udei Bretscher ('99, 

 p. 411), F. hcddardi Bretscher ('00, p. 29), F. macgregori Eisen 

 ('05, p. 118), and F. calif ornica Eisen ('05, p. 119). Because of 

 brevity, use of general terms, and indefinite statements, the descrip- 

 tions of F. udei and F. beddardi are somewhat puzzling and compari- 

 sons troublesome, but the aggregate of differences is such that it is not 

 difficult to separate F. tcnera from them. It differs from F. lohifera 

 in length, and in the characters of the brain, peptonephridia, and 

 spermatheccT ; from F. udei in length, and in the characters of the 

 peptonephridia, spermatheco?, and nephridia; from F. beddardi in 

 the characters of the peptonephridia, brain, and spermathecse ; from 

 F. macgregori in length, number of somites, and in the characters of 

 the brain and spermathecse; and from F. calif ornica in length, num- 

 ber of somites, and the characters of the brain and spermiducal fun- 

 nel. 



In connection with the above comparison a certain discrepancy 

 which appears in Eisen's description of F. macgregori should be 

 noted. In his description of the brain, he says, "Brain anteriorly 

 much convex; posteriorly slightly so." His text figure of the brain 

 shows that the anterior margin is less convex than the posterior. It 

 seems probable that the figure is correct. 



EXTERNAL CHARACTERS 



The worms are relatively slender and delicate, having a length of 

 9-17 mm., and a maximum diameter of only about 0.4 mm. The 

 number of somites varies from 52 to 59. The living worms are 

 opaque and whitish in appearance. The clitellum is on XII and 

 XIII and is only moderately developed. It is made up of transverse 

 rows (PI. XCIX, Fig. 13) in which glandular cells alternate with clear 

 ones. The intersegmental grooves are obscure, except the first four 

 or five. The setje are of the typical Fridericia type and are relatively 

 large. The distal ends are sharply pointed, and the proximal ones 

 (PI. XCIX, Fig. 14) are strongly recurved. In the anterior part of 

 the body the dorsal bundles contain 4-5 set?e, the ventral ones usually 

 6; in the middle region both sets of bundles contain 4 setse; while 

 in the posterior part, 2 is the predominating number. 



