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 Rcesel'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., but 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 figiire 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, 

 ''dorso Isevi jier totam longitudinem nullum carinaj, dentium, spinarumve 

 monstrante vestigium." 



B B 



