Mr. R. I. Pocock on Isometrus americanus {Linn.). 55 



precisely the same characters (and not in others) as do the 

 males just referred to from their females. 



This species is Sc. fbrcipula (Gervais, Arch. Mus. iv. 

 p. 221, pi. xi. fig. 26). Consequently there is no escape 

 from the conclusion that forcipida is as much a synonym of 

 americanus as ohscurus is. But, in addition, forcipula with 

 its wide and excavated fifth caudal seg-ment falls within the 

 definition of the genus Phassus of Thorell — a genus differing 

 from Isometrus apparently ordy in this character. I suspect 

 therefore that columbianiis, the type of the genus Phassus, is 

 a male of some species of Isometrus of which the female is 

 unknown. If this be so, Phassus can scarcely be recognized 

 as a genus, unless, indeed, one goes to the extent of kee[)ing 

 it for those species of Isometrus in which the sexes differ as 

 do those of americanus. 



If the conclusions here set forth are valid the synonymy of 

 Isometrus americanus will be as follows : — 



Isometrus americanus, Linn. Mus. Adolph. Frid. p. 84 

 (1754), ?? 



europaus, Linn. Syst. Nat. ed.lO, p. 625 (175S), ? ?j 



De Geer, Mem. vii. p. 344, pi. xli. figs. 5-8, ? . 



ohscurus, Gervais, Arch. Mus. iv. p. 219, ? . 



Phassus forcipula, Gervais, /. c. p. 221, (^ . 



But this conclusion with regard to the sexes of this species 

 by no means agrees with that of Dr. Karsch (Mitth. Miinchn. 

 ent. Ver. 1879, p. 113). 



This author, who appears to be well acquainted witli Is. 

 americanus, asserts that the males may be distinguished 

 from the females by the length of the hand and fingers with 

 reference to the first two caudal segments. In the female, in 

 short, the hand and fingers are considerably longer than these 

 caudal segments ; in the male they are equal to them in 

 length — characters which do not obtain in the sexes as recog- 

 nized by me. 



But in the collection of the British Museum there are a 

 number of specimens of Isometrus which agree sufficiently 

 well with each other to be ranked as the same species, and 

 which at the same time may be divided into two groups 

 upon certain undoubtedly sexual features. These features 

 are precisely those which Dr. Karsch has pointed out as 

 distinctive of the sexes of americanus. Some of these 

 specimens having a short tail and lobate pcctines are un- 

 questionably females; others having a long tail and simple 



