446 PROCEE'DI!N)GS OF THE NAflTOlSrAI* MUSEUM vol. 91 



In his studies on the North American crayfishes of the genus Cam- 

 harus^ Dr. Herman A. Hagen remarked, according to Faxon (1885, 

 p. 17) : "If the reader is unable to determine * * * the speci- 

 mens in his hands * * * through lack of males, the fault lies, 

 * * * not in the principle of classification, but in the scantiness 

 of his material. A species involves two sexes; and until the species 

 is known, it avails little to attempt the determination of a specimen 

 in this difficult genus." ^ 



Aegla, likewise, is a difficult genus. Certain forms represent un- 

 questionably distinct species; others have been proposed with some 

 hesitation ; two have been rated merely subspecies. 



For the present, at least, it has been necessary to confine specific 

 descriptions and diagnostic key characters to as fully developed male 

 specimens as it has been possible to obtain, for in the females the 

 specific characters do not seem to come to full fruition, and with 

 only females at hand it may be difficult or perhaps at times impossible 

 to identify them as to species. 



In Aegla, the female, in some respects at least, is definitely the 

 weaker sex, and, even if attaining as large a size, it is never so 

 distinctively developed specifically as the corresponding male. This 

 is particularly true of the hands, or chelae. In either sex these are 

 sufficiently asymmetrical to be referred to as the major and the 

 minor chela. The larger chela may be either the right or the left 

 one, but it is usually the left hand, with comparatively few excep- 

 tions, that is the larger. The chelae in the female are undersized 

 and underdeveloped, more of the pattern of the minor chela of the 

 male, which, in turn, might be described as being more or less 

 feminine in appearance. The hands or chelae of the males, more 

 especially the larger one, tend to become more and more swollen as 

 the animals get older and larger. 



The prehensile margins of the fingers are furnished with a close- 

 set pavement or palisade of corneous scales; this armature is not 

 otherwise mentioned, although the presence or absence of a large, 

 usually conspicuous, "lobular" tooth is mentioned in the descrip- 

 tions of certain species and in the diagnostic key. A tooth of this 

 nature occurs on the prehensile margin of the fixed finger of the 

 major chela of most species, usually on the corresponding finger 

 of the minor chela also; often the movable finger has a somewhat 

 similar tooth opposed to one on the fixed finger. In three species 

 the prehensile margin of the fixed finger is without such a lobular 

 tooth: A. sanlorenzo, A. jujuyana, and A. humahuaca. 



■' Specimens studied should be of reasonable size and development. It is difficult to deal 

 with specimens of less than 20 mm. in length of carapace and rostrum together and, indeed, 

 even slightly larger individuals are often none too well developed, even though male. 



