8 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



ment. The first pleopod of the male bears a close resemblance to P. 

 mexicanus (Erichson, 1846, p. 99) and its allies, and, since the current 

 definition of the genus Procambarus need not be modified to include 

 this species, I am assigning it somewhat arbitrarily to Procambarus. 

 In so doing, consideration has been given to this combination of 

 morphological features in a crayfish situated in a geographic area 

 that seems to be crucial from a phylogenetic standpoint (see below). 

 Suffice it to say that, regardless of the genus to which it it assigned, 

 the unique first pleopod of the male and annulus ventralis of the 

 female will serve to distinguish it from any other described crayfish. 



Phylogenetic considerations (figs. 13-21). — There is nothing 

 extraordinary about certain species of closely related genera sharing 

 characteristics of both genera; in fact, the existence of such species 

 provides the best morphological evidence, in the absence of a fossil 

 record, for conclusions that the genera share common ancestry. In 

 the case of the crayfish genera Orconectes and Procambarus, several 

 able students (Ortmann, 1905a; Rhoades, 1962; and Fitzpatrick, 

 in press) have suggested that the former assemblage of species has 

 had a common origin not too remote from Procambarus, and most, 

 if not all, present workers would concede a derivation of Orconectes 

 from some Procambarus stock. Largely on the presence of hooks on 

 the ischia of the third and fourth pereiopods, Faxon (1885, p. 18) 

 referred 0. p. pellucidus [=Cambarus pellucidus] to his Group I, 

 which encompassed a large segment of the present genus Procambarus. 

 Ortmann (1905a, p. 107) was aware of the Procambarus-Yfee traits 

 of 0. pellucidus but argued that it should be included in the genus 

 Orconectes [=his subgenus Faxonius]. 



Although no one has so stated, Hobbs (1942, p. 342) and Villalobos 

 (1955, p. 46) realized the difficulty of defining the genus Procambarus 

 in such a way as to include P. mexicanus and most of its close relatives. 

 Without the statements "if pleopods terminate in two parts, shoulder 

 always present" in the diagnosis of Procambarus and "Never is a 

 strongly developed shoulder present on the cephalic margin near the 

 tip of the appendage" in the diagnosis of Orconectes, all but one mem- 

 ber of the Mexicanus Group would have to be included in the genus 

 Orconectes. As is implied, the key generic differences between members 

 of the two genera exist in the first pleopod of the male. All but one 

 member of the genus Orconectes, 0. p. australis (Rhoades, 1941, 

 p. 142), possess a first pleopod ending in two parts, whereas few 

 species of Procambarus have such a pleopod. Most members of the 

 latter have at least three terminal elements on their first pleopod, 

 and the majority, four or five. For this and other reasons, Hobbs 

 (1962, p. 278) postulated that the pleopod of the ancestral form of 

 the genus Procambarus terminated in four distinct parts. 



