392 GAMMARTD^. 



animals now found in it, (and which are known at this 

 day only to inhabit the extreme north), could have lived 

 in the same latitude as the south of Sweden. The 

 evidence of these fresh-water lakes suggests that similar 

 changes in the relative position of sea and land may have 

 been the cause of our having fresh-water Crustacea nearly 

 allied to marine species in our rivers and inland streams. 

 A fact, however, recorded by Hosius with regard to the ha- 

 bitats of the three fresh-water species of western Germany 

 would seem to discountenance the idea that these animals 

 can adapt themselves to any great change of situation. 

 " Inter loca, quae habitant, multum interesse mihi vicletur; 

 G. puteanus [Niphargus p.~\ tantum in puteo, G. pulex non 

 nisi in rivis rapide profluentibus atque minime profundis, 

 G. fluviatilis [Rosel's species] in aquis solum stagnanti- 

 bus aut lente manantibus ac profundis versatur. Nun- 

 quam hae ultimas species una mihi occurrerunt, nee si aquas, 

 in quibus co?nmorantur, inter se conjunct^ erant. Dum 

 enim, ut exemplum afferam, in piscina Poppelsdorrfiana 

 G. fluviatilis solus reperitur, in rivo, qui e montibus 

 hanc in piscinam effunditur, solus G. pulex versatur. 

 Idem quoque mihi aquas prope Godesbergam perscrutato, 

 probabile videbatur, ubi rivus, qui a Marienforst in 

 planitiem defertur, tantum G.pulicem colit, duminstagnis 

 cum rivo cohserentibus solus G. fluviatilis versatur." 

 Dissert, de Gammari Spec. p. 18. 



Taking this statement of the diversity in the habitats 

 of the two species of Gammarus, as here restricted, 

 into consideration, we anticipate that Rosel's tooth- 

 backed species will yet be found in some of our 

 ponds and stagnant waters, although it has not hitherto 

 been detected. We are also hence enabled to identify 

 the descriptions of various writers, from the habitat in 

 which the specimens were found ; thus, where Latreille 

 says that his Gammarus pulex is found, " dans Peaux 



