GAMMARUS. 
369 
identical with G. fluviatilis of Milne Edwards. That 
author has, however, described a second French fresh- 
water species, under the name of G. pulex , which he 
places in his division A. a a ., and which he describes as 
entirely destitute of teeth or spines on the back of the 
tail. We have seen no species of Gammarus agree- 
ing with such a description ; it certainly does not agree 
either with the figures of Geoffroy (who, in fact, copies 
Roesel’s figure of the tooth-backed species), or of Zenker, 
who as certainly figures our common British species with 
the first, second, and third segments of the tail, unarmed, 
and with the fourth, fifth, and sixth furnished with small 
spines.* The species represented in Roesel’s figures (con- 
firmed as they are in the details given by Hosius) and 
which M. Gervais has named G. Roeselii, ought to enter 
Milne Edwards’s section B., hut the species of which he 
has formed that section constitute the several genera 
Amathilla , Gammar acanthus , and a doubtful Gammarus . 
Dr. Leach adopted at different times different charac- 
ters for the distribution of the species, at one time 
forming his primary section according to the fresh water 
or marine habitats of the species, and at another time 
dividing them according to the rudimental or developed 
condition of the inner branch of the last pair of append- 
ages of the tail. Without establishing distinct divisions 
in the genus, we have adopted the latter character as 
the ground for our arrangement of the species, com- 
mencing with those in which the inner branch is almost 
obsolete, and terminating with those in which it is as 
large as the other branch. 
* Milne Edwards, however, appears to be confirmed in this character by 
the figure which he has given of G. Othonis, which has the tail entirely 
unarmed with teeth or spines, as well as by Kroyer’s description of G. pinguis, 
1 ‘ dorso lsevi per totam longitudinem nullum Carinas, dentium, spinarumve 
monstrante vestigium.” 
B B 
