48 



Tlie number of species of laud mollusca now known from 

 the region of tlie northern rivers is eight, distributed generic- 

 ally as follows : — Helix six, JBulimus one, and Stenogyra one. 

 Six of these are either specifically identical or closely allied to 

 extra-limital species. The Helices are considered peculiar to 

 the country, but with the exception of two of them no critical 

 comparisons seem to have been instituted. H. pommn, the 

 most widely diffused, is very closely related to H. facJiystyla 

 of N.E. Queensland ; and R. Meadei and H. Lyndii are also 

 near allies. H. prumtm has an analogue in H. argillacea of 

 Timor, but on the other hand, it is questionably distinct from 

 S. Coxeni from Whitsunday Island, off Port Denison, Queens- 

 land ; and H. Bennetti from Ipswich, Queensland, is another 

 critical species of the same group The facts are few, but 

 almost warrant the inference that the Northern Territory 

 Helices are forms of Queensland species modified in the course 

 of their migration. The other land pulmonates are Bulimus 

 JBeddomei, previously known from Torres Straits, and Stenogyra 

 Tuckeri, of Polynesian, if not of American origin, and probably 

 xin alien. 



The freshwater shells are fairly well represented specifically, 

 and include a greater number of endemic species than might 

 have been expected. Almost every perennial water-course or 

 lagoon has its moUuscan denizens. But just as there are two 

 well-marked geographical regions in tropical South Australia, 

 80 do we find a corresponding difference in the freshwater 

 shells of the two regions. 



The central portion of the continent yields Unio Stuarfii, 

 JPaludina WaterTiousei, and P. Kingii. 



The basin of the Roper, JJnio Angasi; that of the Ade- 

 laide, several LimncBa^ Bulinus^ Planorhis, Paludina, Unio^ 

 Corhicula, and one species of each of Physopsis, Ancylus, and 

 Neritina; that of the Lower Victoria Eiver, Melanice, a 

 Baludina, a Bulinus, a Bitliinia, a Corhicula, and a Mycetopus, 

 nearly all specifically peculiar. 



Several of the species have a wide range throughout the 

 eastern half of the continent, and, with two exceptions, the 

 genera are of world-w^de distribution. Mycetopus has its head 

 quarters in tropical America, but a species is known from Siam. 

 Physopsis includes another species belonging to South Africa. 

 The Australian representatives of these genera are confined 

 within narrow limits. Ifycetopns in one species is only known 

 from the lower Victoria River, and Physopsis from the basin of 

 the Adelaide Eiver and Coburg Peninsula. 



