142 J. F. Fitzpatrick, Jr. 



short time, it becomes a very interesting subject for dispersal and 

 competition studies. The question is complicated by the presence of the 

 relatively primitive P. (S.) strenthi Hobbs (1977) in San Luis Potosl, 

 Mexico. 



Numerous other problems exist in the undiscussed subgenera of 

 Procambarus. But the purpose of this treatment is not to attempt an 

 exhaustive resolution of zoogeographical situations of North America. 

 Instead, it is to emphasize that more sophisticated knowledge of the 

 geology of an evolutionary critical area can and does require careful 

 reflection on prior conclusions with respect to the phylogeny of the 

 animals, and especially the temporal assignments of events. Thus, the 

 specific answers are best left to other studies. 



The discussion would not be complete, however, without some 

 mention of the genera Cambarus and Orconectes. As noted above, they 

 both are poorly represented in the area of the old Mississippi 

 Embayment. Until more is known of the precise relationships of the 

 several populations of Cambarus {Lacunicambarus) almost nothing can 

 be said of their history. This was recognized by Hobbs (1969), and the 

 only progress thus far has been the description of two restricted, 

 peripheral species (Fitzpatrick 1978, Hobbs 1981), leaving all the principal 

 questions still unanswered. Otherwise, only C. (Depressicambarus) 

 striatus, an "advanced" member of a "relatively primitive" group, invades 

 to the Mississippi River. Particularly important here are the habits of 

 this species. I have observed individuals moving across open ground 

 when the humidity is only moderately high, and I have found their bur- 

 rows on hillsides somewhat removed from flowing or standing surface 

 water. Surely, this species is not as restricted in its dispersal as are many 

 others. 



Orconectes is represented by no relatively primitive species. Although 

 the exact relationships of the taxa are presently undetermined, I am 

 sufficiently progressed in a monographic study of the genus to be com- 

 fortable with the concept that the area in question is populated by rela- 

 tively advanced forms. Many are members of the Palmeri Group; they 

 probably represent an invasion from the west. Most of the remainder 

 are Virilis Section species, which probably represent an eastern assem- 

 blage expansion. The striking feature is the absence of simple, less 

 advanced forms. 



Hobbs's (1967, 1981, 1983) arguments in favor of an early diver- 

 gence of procambarid-like stocks are quite sound. Equally, his ideas of 

 the emergence of proto-Cambarus and proto-Orconectes cannot be 

 faulted. The paradox exists in the geologic data that suggest a large 

 Midway-time river from the southern Appalachians, entering the Mis- 

 sissippi Embayment in the area near the headwaters of the present Pearl 

 River (Grim 1936). Another strong river reworked the "Citronelle" depos- 

 its and emptied just north of Lake Pontchartrain (Brown 1967). Current 

 dating would place these events in late Miocene or early Pliocene. 



