4 PROCEEDINGS OF THE NATIONAL MUSEUM VOL, 126 
to preserve nomenclatural stability. The genus Cambarincola, as 
presently understood, is composed of, by far, the largest number of 
species of any genus of the order Branchiobdellida. With the exception 
of the Eurasian genus Branchiobdella, it has the greatest geographical 
range of any genus now known. In almost all localities where they 
occur, the species of Cambarincola are the dominant elements of the 
branchiobdellid fauna. To transfer all these species to the much less 
well-known and smaller genus Pterodrilus could result only in a period 
of nomenclatural confusion. 
This argument alone, however, cannot justify excluding the species 
now assigned to Cambarincola from Pterodrilus. Moore was struck with 
the unusual appearance of P. alcicornus and P. distichus with their 
ornamentation of dorsal projections, and he established a new genus 
for them. Although he described the male reproductive system of both 
species (Moore, 1895a, pp. 453, 454), the importance of this system 
to the systematics of the branchiobdellids was not appreciated at that 
time, nor, indeed, by Ellis who did, however, present a diagram of it 
in his paper establishing the genus Cambarincola (Ellis, 1912, p. 483). 
The difficulty arises from the fact that the basic plan of the repro- 
ductive system of species of Pterodrilus does not differ from that of 
the members of Cambarincola as much as it does from other genera 
of the order. Recent workers (Holt, 1960a, 1960b, 1967a, 1967b, 
Hoffman, 1963; Laing, 1963) have derived their generic concepts from 
the major variations in pattern of the male reproductive system, and 
I regard these variations as furnishing the most usable characters for 
marshalling groups of species into genera. Also, the jaws of species of 
Pterodrilus are quite similar in shape and arrangement of the teeth 
to those of species of Cambarincola. But the jaw patterns are shared 
by two or more genera in other cases, and the jaws of all species of 
Pterodrilus are of essentially the same form. 
We have, however, in Pterodrilus a group of distinctive species that 
obviously belong together as a specialized offshoot from the main 
direction of the evolution of Cambarincola. A formal diagnosis obscures 
by its brevity and technical language the distinctiveness of such a 
group. The species of Pterodrilus are smaller than those of Cambarin- 
cola and are characteristically delicate in appearance. The jaws are 
correspondingly reduced in size and pigmentation. Always there are 
ridges on some of the segments and usually these ridges bear projec- 
tions. It is true that both segmental ridges, produced by supernumer- 
ary muscles (Holt, 1960b, p. 172), and projections of various sorts 
occur in other genera and that several species of Cambarincola have 
such ridges. None of the latter species, however, are easily confused 
with those of Pterodrilus. The male reproductive systems of species of 
Pterodrilus vary, but the spermiducal gland is relatively short and 
