10 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



the absence of a proannular plate in the females. It does seem probable, 

 nevertheless, that the primitive Orconectes did have a more recent com- 

 mon ancestry with the members of the Mexicanus Section than with 

 other segments of the genus Procambarus and that Procambarus pecki 

 also shares in this ancestry. 



It is postulated here that the ancestral stock from which originated 

 P. pecki, the Mexicanus and Cubensis Groups of the genus Procambarus, 

 and the genus Orconectes possessed, among others, the following char- 

 acteristics: (1) a long rostrum with prominent marginal spines and 

 acumen; (2) a broad and short areola; (3) multiple (more than one) 

 cervical spines on each side; (4) subcylindrical to subovate chelae 

 with elongate and entirely tuberculate palms; (5) symmetrical first 

 pleopods in the first form male with four short terminal elements, 

 the cephalic process reduced, and with a shoulder distinctly proxi- 

 mally removed from the base of the central projection on the cephalic 

 surfaces of the appendages (fig. 14) ; (6) hooks on the ischia of the 

 third and fourth pereiopods in the male; and (7) an inflated, oval 

 annulus ventralis without a median depression, and with a short 

 arclike sinus on the caudal declivity. 



In Procambarus pecki, the primitive characteristics of the stock are 

 maintained in the rostrum, cervical spines, and chelae. The areola has 

 become elongated; the first pleopod has lost both the cephalic and 

 caudal processes; hooks are present on the ischia of only the third 

 pereiopods; and the annulus ventralis has developed a prominent 

 median longitudinal depression. 



Utilizing a combination of the most primitive characters represented 

 in the members of the Mexicanus Section, it is postulated that the 

 stock moving into Mexico, and subsequently into Cuba, retained the 

 primitive condition of the rostrum, areola, cervical spines, and chelae, 

 and modified the first pleopods as depicted in figure 14. It lost, how- 

 ever, hooks on the ischia of the fourth pereiopods, and the females 

 developed a prominent sternal plate, a proannular plate, immediately 

 cephalic to the annulus ventralis. 



Again, combining the most primitive characteristics represented in 

 the Limosus Section of the genus Orconectes, their ancestors retained 

 the primitive rostrum, areola, cervical spines, hooks on the ischia of 

 the pereiopods, and annulus ventralis. The major alterations were in 

 the chelae, in which the uniform tuberculate condition was reduced 

 largely to two rows along the upper inner margin of the palm, and in 

 the first pleopod, in which the cephalic process was lost completely 

 and the caudal process reduced to a rudimentary spine (fig. 19). 



To a considerable degree, along with divergence, parallel evolution 

 has progressed in the evolution of P. pecki, the primitive Orconectes, 

 and the Mexicanus stocks. In general, in all three the areola has under- 



