ATYOIDA POTIMIRIM, A MTJD-EATING FRESHWATER PRAWN. 335 
not only served as a certain character for species, but it has 
even been employed for the distinction of genera. Thus, the 
genus Leancler has been separated from Palcemon solely because 
the former possesses a ‘ spina branchiostegiana,’ hut no ‘ spina 
hepatica ; ’ and the latter a ‘spina hepatica/ hut no ‘spina 
branchiostegiana.’ 'Now, in Atyoida Potimirim the presence or 
absence of the ‘ spina pterygostomiana ’ constitutes a simple dis- 
tinction of sex or age, for all the younger females of less than 
12 millimetres long agree with the males in wanting the spine 
at the lower angle of the carapace. As the males remain much 
smaller than the females, it seems quite intelligible that many 
peculiarities of the females, which only make their appearance 
when the latter have grown to exceed the size of the males, 
may never arrive at development in the latter. In this way, 
for example, we may explain the difference in the number of 
bristles at the extremity of the last segment of the body, or the 
middle caudal plate ( Telson ) in adult males and females, as this 
number in general increases with size and age, although it is 
not always the case that larger animals have more numerous 
bristles. In eleven young animals, 8-12 millimetres long, 
which as yet showed no external sexual differences, I found 
from 6 to 9, or an average of 7*1 bristles ; ten males of 12-13 
millimetres had 8-10, or an average of 8*7 bristles ; in eleven 
males of 14-15 millimetres, the number of bristles varied be- 
tween 8 and 10, but the average amounted to 9*9 ; in nine 
females of 12-19 millimetres there were 8-14, average 10 '6 
bristles ; and lastly, in fourteen females of 20-24 millimetres, 
from 12 to 17, or an average of 14’4 bristles, were present.* 
It would be equally easy to correlate the absence of the 
tooth at the lower angle of the carapace in the males with their 
smaller size, if it were not that in females of 13-15 millimetres 
in length, which consequently do not yet exceed the adult males 
in size, the tooth is already at all events indicated, and in 
general even developed to its full size. Under these circum- 
stances the most probable supposition seems to me to be that 
the ‘ spina pterygostomiana ’ first appeared in adult, or nearly 
adult, females, and in course of time was produced in younger 
and younger individuals of the same sex. However this may 
be, we have here a notable exception to the almost universal 
rule that the males depart further than the females from the 
common youthful form. 
As our Atyoida differs in a whole series of peculiarities from 
the ordinary structure of the Prawns, such as Palcemon, Hippo- 
* This number of bristles, which varies so much in Atyoida Potimirim, has 
been adopted by Heller in the case of the nearly allied genus Caridina among 
the generic characters; he ascribes to it nine setse (Heller, Crustaceen des siid- 
lichen JEuropa, p. 238). 
