no. 3621 CRAYFISH FROM ALABAMA CAVES — HOBBS 13 



of the hooks on the ischia of the third and fourth pereiopods is main- 

 tained only in C. dissitus Penn (1955, p. 73) (Louisiana) and the 

 undescribed species mentioned above; all others have them only on 

 the third pereiopods. The annulus ventralis in all species has elements 

 of the median longitudinal trough; thus, the same trends that were 

 pointed out for Orconectes and the Mexicanus Section of Procambarus 

 and that were attained in P. pecki are apparent in the evolution of 

 the genus Cambarus. 



Figures 13 through 20 depict a hypothesis relating the ancestry of 

 the genera Orconectes, Faxonella, and Cambarus, the Mexicanus, 

 Barbatus, and Gracilis Sections of the genus Procambarus, and the 

 disjunct P. pecki and P. alleni (Faxon, 1884, p. 110). Figure 13, rep- 

 resenting the generalized first pleopod of Procambarus is redrawn 

 from Hobbs (1962, p. 275) ; references to the original figure will enable 

 the reader to associate the species groups discussed here with other 

 segments of the genus Procambarus. 



The chief pre- or early-Miocene changes postulated in the evolution 

 of the first pleopod of the males of this stock (fig. 14) are (1) the 

 accentuation of the shoulder on the cephalic surface, (2) a narrowing 

 of the cephalic-caudal plane in the distal portion of the appendage, 

 (3) the beginning of an elongation of the caudal knob, and (4) a re- 

 duction in the size of the cephalic process and its becoming more in- 

 timately associated with the central projection. 



In the Mexicanus-Cubensis stock (fig. 16), the distal portion of the 

 appendage became elongate and slenderer, thus accentuating the 

 shoulder; the cephalic process and central projection were reduced in 

 size, and the subapical setae lost. 



In P. alleni (fig. 17), the distal portion of the appendage became 

 even more attenuate than in the Mexicanus-Cubensis stock, and the 

 central projection disproportionately smaller than the cephalic process. 

 The subapical setae were retained and extend along the margin of the 

 rounded shoulder. 



In the Barbatus-Gracilis stock (fig. 15), the distal portion of the 

 appendage was shortened, bringing the terminal elements and the 

 subapical setae to the level of the cephalic shoulder. The subterminal 

 setae were subsequently lost in the members of the Gracilis Section. 



In the Orconectes-Faxonella stock (fig. 19), the cephalic process and 

 subterminal setae were lost, the caudal element reduced to a vestige, 

 the mesial process shortened to project little farther than the central 

 projection, and the latter somewhat broadened in the cephalic-caudal 

 plane. 



The Cambarus stock (fig. 20) lost the cephalic shoulder and sub- 

 apical setae, markedly reduced the caudal element to a small knob, 

 shifted the cephalic process to a mesial position at the morpholog- 



