12 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



gone a lengthening and narrowing; only in P. atkinsoni (Ortmann, 

 1913, p. 414) (Isle of Pines) is the primitive condition retained. There 

 has been a tendency to reduce the number of cervical spines to a 

 single one so that the multiple condition persists only in P. pecki, 

 0. pellucidus (Kentucky to Alabama), 0. limosus (Rafinesque, 1817, 

 p. 42) (eastern United States), P. acanthophorus, P. llamasi Villalobos 

 (1954, p. 364), P. pilosimanus (Ortmann, 1906, p. 6) (Guatemala to 

 southern Mexico), and P. williamsoni (Ortmann, 1905b, p. 439) 

 (Guatemala), and it has been reduced to a small tubercle or completely 

 obliterated in 0. pellucidus testii (Hay, 1891, p. 148) (Indiana) and 

 in several of the Mexican species. A reduction in the number of terminal 

 elements of the pleopod is a characteristic of the three; among the 

 species of the genus Orconectes, all have only the mesial process and 

 central projection except 0. pellucidus australis (Alabama and Ten- 

 nessee), which has in addition a minute caudal process; in P. pecki 

 no traces of the cephalic and caudal processes are present; in the 

 members of the Mexicanus Section, the cephalic process is retained 

 only in P. acanthophorus, and the caudal process proper only in the 

 Cuban forms. Hooks on the ischia of the pereiopods have been reduced 

 to those on the third in all except 0. pellucidus and 0. quadruncus 

 (Creaser, 1933, p. 10) (Missouri). The primitive, ovate, swollen annulus 

 ventralis has developed a groove similar to that in P. pecki, both in 

 most of the Mexican representatives of the Mexicanus Section and 

 in the genus Orconectes. 



To a lesser degree, evolution in the genus Cambarus has paralleled 

 that of the three mentioned above. While some of the more primitive 

 species have retained the broad, short areola, in most it has been 

 narrowed and lengthened. The multiple cervical spines are retained 

 only in the troglobitic G. hamulatus (Cope and Packard, 1881, p. 881) 

 (Tennessee) and C. setosus (Faxon, 1889, p. 237) (Missouri) ; even in 

 some specimens of these species the number has been reduced to one, 

 and the one sometimes represented by a tubercle. The primitive 

 pleopod of Cambarus (fig. 20), like that of the primitive Procambarus, 

 possessed four terminal elements, but the cephalic process had shifted 

 to a position mesial to the shaft and is known to exist in only two 

 extant species, C. strawni Reimer (1966, p. 11) (Arkansas) and in a 

 species from Louisiana, the description of which by Black is in press. 

 The caudal element is reduced to a small caudal knob that is best 

 developed in C. pristinus Hobbs (1965, p. 268) (Tennessee), but 

 vestiges of it occur in several other species of the genus; in most, 

 however, it is absent. Most species of the genus may be characterized 

 by possessing a first pleopod with only two terminal elements (mesial 

 process and central projection) that are directed approximately at 

 a right angle to the shaft of the appendage. The primitive condition 



