56 Mr. R. I. Pocock on Isometrus 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 
fingers. 
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, ¢. 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 /sometrus 
americanus present themselves under two very different 
aspects. But, 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.). 
& ¢.—Inferior caudal keels not confluent. 
2 .—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. 
3 .—Pectines not lobate. 
Tail not more than six times as long as cephalo- 
thorax, manifestly thicker towards the middle of 
the fitth segment, then abruptly narrowed. 
Brachium only about two thirds the width of the 
