14 U.S. NATIONAL MUSEUM BULLETIN 2 59 



the antennae and pereopods. Allometric size differences, however, 

 were more commonly expressed in larger males than in larger females, 

 but there were a number of exceptions. 



On the basis of examined material it was concluded that in some 

 species mature males are characteristically smaller than mature 

 females, while in other species the reverse is true, and, finally, in a 

 few species there may be no real difference in size between the sexes. 

 As already mentioned, however, determination of sexually mature 

 males was frequently difficult, but sexual maturity could usually 

 be estabhshed if sufiicient comparative material was available. In 

 several species, secondary sex characters were pronounced in the 

 male; thus with these species sexual maturity was comparatively 

 easy to ascertain, although almost invariably these characters were 

 more extreme in older males. It is apparent in the males of a number 

 of species of Stygonectes that appendages (especially antennae and 

 pereopods) grow allometrically in relation to the body and in some 

 instances in relation to each other. In species where allometric 

 size relationships were obvious, structural variation in the appendages 

 of larger specimens was given careful consideration when these 

 structures were utilized in dehneating diagnostic characters. 



In only a few instances was it necessary to base descriptions or 

 diagnostic characters on what appeared to be slightly immature speci- 

 mens, and where such action was necessitated, it is so indicated. 

 Ideally, where this has occurred, future collecting will provide sexually 

 mature specimens of these species, at which time their diagnostic 

 characters can be reevaluated and brought into fine with those spe- 

 cies in the genus described from mature individuals. 



Acknowledgments 



A number of people have assisted me in this study and to each of 

 them I am deeply indebted. I am especially grateful to Dr. Thomas 

 C. Barr, Jr. for his encouragement, patience, and advice and for giving 

 me a free choice in my research. The cooperation of Mr. LesUe 

 Hubricht in making available his large personal collection of subter- 

 ranean amphipods for this investigation is greatly appreciated. I am 

 grateful to Dr. Thomas E. Bowman of the United States National 

 Museum and Dr. E. L. Bousfield of the National Museum of Canada, 

 both of whom lent encouragement and advice and made available 

 amphipod collections from their respective museums. In addition, 

 I had several instructive discussions with Dr. Bowman during the 

 development of this research. I also thank Dr. WiUard D. Hartman 



