1905.] ORTMANN — AFFINITIES OF CAMBARUS. 93 



specialized types (C. gracilis and allies), and even some of the 

 primitive forms (C. cubensis) possess some peculiar and apparently 

 advanced characters. I think we can express it this way : among 

 the second group, there are species that approach most closely the 

 original, stock of the genus, but they themselves are modified to a 

 degree. If I am to single out a species that possibly is the most 

 primitive, I should name C. digueti Bouvier (Bull. Mus. Paris, 

 1897, p. 224), which is identical with C. carinatus Faxon (Pr. 

 U. S. Mies., 20, 1898, p. 648). It is remarkable that the most 

 primitive forms are found in Mexico (and Cuba), which agrees 

 well with the theory of the origin of the genus in these parts (see 

 Ortmann, /. c. , 1902, p. 283). 



CHARACTERS OF THE GENUS CAMBARUS THAT SERVE TO 

 DISTINGUISH GROUPS AND SPECIES. 



Sexual Organs. — Already the earlier writers (Girard, Hagen) 

 have pointed out the importance of the anterior pair of abdominal 

 appendages of the male (copulatory organs) for systematic pur- 

 poses. Faxon made large use of them in defining his five groups, 

 but within the groups he rarely tried to avail himself of these limbs 

 to reveal the mutual affinities of the different species. It is now 

 generally known that these organs furnish not only the best specific 

 characters, but that their similarity in certain assemblages of species 

 clearly indicates genetic relationship. In close connection with 

 the shape of this organ is that of the female " annulus ventralis," 

 as we now know, the receptaculum seminis. There is, however, 

 not so much variety in the shape of the latter organ, it is not so 

 polymorphous, and the main types occur in different groups, which 

 is apparently due to the more simple structure of this organ. 

 Nevertheless, the annulus — in connection with the male organ — 

 is rather important. We may add that in a number of species 

 the annulus is not very well known. 



As regards the male organs, their shape is very complex and 

 much varied. Several main types may be distinguished, and these 

 again show much diversity. It is hardly likely that in so complex 

 an organ the identical form has developed several times, that is to 

 say that there are cases of parallelism ; where there is identity or 

 similarity of these organs in different species, this is generally and 

 surely a sign of close affinity. Only one or two exceptions in the 



