294 PROCEEDINGS OF THE NATIONAL MUSEUM vol. ii4 



segments v and vi testicular, and the male sex organs consist of the 

 elements listed in the preceding paragraph. Discounting poorly 

 described species, there are about 18 forms, which Goodnight placed 

 in the 3 genera Pterodrilus, Cirrodrilus, and Cambarincola. Of these 

 three, Cirrodiilus has been disposed of (Holt, 1960a) in a careful paper 

 which restores the correct name Ceratodrilus to the American species 

 and elucidates tlie anatomy of the genus. This genus differs from 

 aD others in the collective characters of (1) eversible penis, (2) virtual 

 absence of the prostate gland, (3) great elaboration of segmental and 

 peristomial epidermal tentacles. 



Cambarincola in the usage of Ellis and Goodnight is of course 

 heterogeneous. The genus in the strict sense includes only those 

 species which do not have segmental ornamentation, in which the 

 bursa, but not the small penis, is eversible (or extrusible), the ejacula- 

 tory duct is not strongly modified, and the prostate gland is present 

 and functional. This combination of characters rules out three 

 known species originally described in the genus: inversa Ellis, elevata 

 Goodnight, and macbaini Holt. Each of the last two named repre- 

 sents a distinct generic type, now being defined and readied for publi- 

 cation by Professor Holt. We have studied the tj^pes of C. inversa 

 and regard it, too, as worthy of generic rank, but action is deferred 

 pending the acquisition of fresh material for sectioning. 



Pterodrilus is hkewise heterogeneous. The species alcicornus and 

 distichus (with two additional undescribed forms) are basically very 

 similar internally. P. mexicanus is still unkno^vn save from the 

 poorly preserved tj^pe specimen. But P. durbini of Ellis is remarkably 

 dift'erent, and belongs to the recently described genus Ellisodrilus 

 (Holt, 1960b), along with a second, previously unknown related species, 

 E. clitellatus. 



With all of these eliminations and realinements which have resulted 

 from a close comparative study of important internal organs, we are 

 left with a genus Cambarincola in a strict sense (which nonetheless 

 is now laiown to contain no less than 21 species), and a genus Ptero- 

 drilus with two known and two more undescribed forms; and insofar 

 as internal structure is concerned, these two genera might be con- 

 sidered identical. 



Here the element of arbitrary decision has its horn* upon the stage. 

 Some authors who may work wdth branchiobdellids at a future time 

 will perhaps desh'e to combine the two genera under the older name 

 Pterodrilus. My personal feeling is that although the two groups 

 are beyond peradventm-e very closely related, the evidence suggests 

 that the species of Pterodrilus represent an extreme specialization 

 of some early embranchment of the Philadelphica section of Camba- 

 rincola. Evolutionary recency is suggested by the small size of the 



