720 MR. s. KEMP ON DECAPOB [June 15, 



genera, Benthesicymiis, Gennadas, and Amalopenceus, the first 

 being the most primitive and the last the most highly specialized. 



The species of Ge^madas and Amalopenceics are by no means 

 easy of determination. The best characters are undoubtedly 

 those afibrded by the membranous expansion of the endopods of 

 the first pleopods of the male (known as the petasma) and by the 

 sternal plates of the cephalothorax (the thelycum) in the 

 female. 



These characters are of course of little value in very young 

 specimens, but they appear to afibrd trustworthy indications in 

 all examples upwards of one half the maximum length of the 

 species. 



So far as at present known there is extremely little variation 

 in the form of the adult petasma, but this is not necessarily the 

 case with the thelycum, for Bouvier (1908, loc. cit.) has instanced 

 several variations in the case of Amalopenceics vcdens, Smith. 

 Although it might be expected that seasonal variations depending 

 on the degree of sexual maturity of the individual would be 

 manifest in both thelycum and petasma, it must be noticed that 

 there is no evidence of this in the case of AmalopencBios elegans, a 

 species of wide Atlantic distribution and of common occurrence. 



Bouvier has indicated other characters of great value in the 

 determination of the different species. Of these, the most im- 

 portant are the form of the antennary and infra-antennary angles, 

 the proportional length of the second and third joints of the 

 antennular peduncle, the proportional lengths of the ultimate 

 joint of the mandibular palp and of the merus, carpvis, and chela of 

 the first three pairs of pereiopods. Valuable information is also 

 afforded by the form of the antennal scale and by the presence or 

 absence of a stout median spine on the first abdominal sternum. 



The s]3ecimens referred by Spence Bate to Gennadas 2yccrvus and 

 G. intermedius are for the most part fairly perfect ; the majority 

 of those recorded in the ' Challenger ' Report are preserved in the 

 British Museum, and the types of both species are extant. The 

 specimens missing are fovir in number, viz. : — 



St. 137. 35° 59' S., 1° 34' E. Recorded as G. intermedius. 



St. 159. S. of Australia. 1 



St. 250. ]Sr. Pacific. v Recorded as G. parvus. 



St. 289. S. Pacific. J 



The result of an investigation of the remaining specimens, 

 seventeen in number, is indicated in the table on p. 721. 



If these results be accepted, it will be seen that Spence Bate 

 was quite as unfortunate in his treatment of this gen vis as Hansen 

 has shown him to have been with Sergestes. 



In the following systematic notes no attempt has been made to 

 correct the many inaccuracies which disfigure Spence Bate's work. 

 The condensed descriptions and figures will, it is hoped, prove 

 sufficient for the recognition of the type species and the two forms 

 described as new. The various shapes assumed by the lobes and folds 



