202 Prof. C. Chilton on Asellus aquaticus. 
external characters unless one has fully adult and perfect 
males when they might be distinguished by the length of the 
second antenne and by the shape of the lateral margins of 
segments 2 to 5 of the pereeon. Many of my specimens are 
immature and in others the antenne are broken, off, and, 
though the Tunbridge Wells specimens showed the lateral 
margins of the perzeon segments as described by Racovitza, 
the difference from the other specimens was hardly sufficient 
to be distinctive by itself. ; 
The following are brief notes on the specimens I have 
examined. In the female from Edinburgh the-inner lobe 
of the first maxilla showed the four sete characteristic of 
A. aquaticus on the one side, while the appendage on the 
other side had only three * ; the second pleopod is circular in 
outline ; the male examined from Edinburgh is evidently not 
fully mature, for the first thoracic leg has the propod only 
slightly triangular, though it is certainly approaching towards 
the outline represented in Racovitza’s figure ; in the fourth 
leg the row of spinules on the carpus is distinctly discon- 
tinuous and contains only a few spines; the first and second 
pleopods show the characters described by Racovitza, the 
exterior margin of the exopod of pleopod 1 being distinctly 
emarginate. 
In a male specimen of Asellus aquaticus, Linné, from the 
River Neckar the first and fourth pairs of legs correspond, 
on the whole, well with Racovitza’s figures and descriptions, 
though the first one is not fully developed, and consequently 
the propod not so distinctly triangular; the first and 
second pleopods are in close agreement with Racovitza’s 
description, the emargination on the external border of the 
exopod being quite distinct. 
Racovitza has examined and identified specimens of 
Asellus aquaticus, Linné, from “ Askam bog (Yorkshire), 
Birmingham,”’ from various localities in France, and from 
Carniola (Adelsberg), while on the testimony: of other authors 
le records it from Norway, Poland, Livonia, Russia, Germany, 
Switzerland, and Greenland. ‘The species is therefore very 
widely distributed. ‘It is this species that has been so well 
described and figured by Sars f. 
* Probably further examination would show that the oral appendages 
in Asellus are liable to a considerable amount of variation, as has been 
shown by Dr. Collinge to exist in the Oniscoidea or Terrestrial Isopoda 
(Journ. Linn. Soe., Zool. vol. xxxii. (1914) pp. 287-293, pls. xx., xxi.). 
+ 1867, ‘Hist. nat. des Crustacés d’eau douce de Norvége,’ p. 93, 
pls. viii, ix., & x.; and 1897, ‘Crustacea of Norway,’ vol. 11. p. 97, 
pl. xxxix, : 
