56 Mr. R. I. Pocock on Isometius americanus [Linn.). 



pectines are unquestionably males. The females are very like 

 the females of americanus^ and can only be distinguished 

 from such female specimens of that species as I have seen by 

 the confluence of the inferior keels of some of the caudal 

 segments. This confluence occurs in the males also and 

 serves, apart from other features, to separate them from the 

 males of americanus. 



It appears then, so far as a conclusion can be drawn from 

 the few specimens that I have seen, that we have here a 

 species distinct from americanus — a species in which the 

 females can only be distinguished from those of americanus 

 by the confluent caudal keels ; whilst the males, in addition 

 to this character, differ from the males of americanus in 

 having a long slender tail, a narrow hand, and contiguous 

 fingera. 



But Dr. Karsch regards this confluence of the keels merely 

 as of a varietal nature — having seen apparently intermediate 

 forms — and has given to the specimens presenting it the name 

 americanus, var. androcottoides. 



If this be so, i. e. if the females of these long-tailed males 

 be not specifically distinguishable from the females of the 

 thick-tailed males, it seems that we have here a remarkable 

 case of dimorphism, inasmuch as the males of Isometrus 

 americanus present themselves under two very different 

 aspects. I3ut, so far as my observations go, there are two 

 species to be dealt with, namely americanus and andro- 

 cottoides ; and it seems to me to be wiser to regard these two 

 as distinct until the alternative hypothesis of dimorphism be 

 more firmly established than it is at present. 



The differences, sexual and asexual, between these two 

 species may be set forth as follows : — 



Isometrus americanus (Linn.). 



(J $ . — Inferior caudal keels not confluent. 

 $ . — Pectines lobate. 



Tail not more than six times as long as cephalo- 

 thorax, parallel-sided or slightly thicker towards 

 the fifth segment. 

 Brachium very slightly thinner than hand ; fingers 

 not sinuate and in contact when closed. 

 (J , — Pectines not lobate. 



Tail not more than six times as long as cephalo- 

 thorax, manifestly thicker towards the middle of 

 the fifth segment, then abruptly narrowed. 

 Brachium only about two thirds the width of the 



