510 ON THE AUSTRALASIAN GUNDLACHIA, 



secondary by a semilunate orifice on the roof of the latter, of 

 which this orifice occu})ies the central fifth ; posterior to this the 

 two shells are soldered together for some distance, the intrusion 

 of the primary shell forming a smooth flat shelf, the edge of which 

 at the orifice is slightly thickened and concave or sinuous. On 

 the left the primary shell actually projects into the chamber of 

 the secondary shell, on the right the wall of the one passes into 

 that of the other. All the interior is smooth, glossy, and sub- 

 nacreous. Total length 3, breadth 2, height i mm. 



Type in the collection of Prof. Tate. 



At present G. petterdi appears to be known, as adult, from only 

 three localities : a small shallow stagnant pool near the First 

 Basin, South Esk River, Launceston, Tasmania (Johnston and 

 Petterd), a hill stream at Mt. Lofty, S.A. (Tate), and a chain of 

 shallow stagnant ponds behind the sandhills at Henley Beach, 

 near Adelaide, S.A. (Adcock, Pulleine, and Hedley). In this 

 latter locality they were associated with Planorhis, Bulinus, and 

 Ancylus, the latter only determined by the shell. Their habit 

 was to cling to drowned leaves and sticks, or to the submerged 

 leaves and stems of water plants. So closely do they resemble 

 Ancylus that a careful observer may, in the field, easily mistake 

 one for another. 



The precise mode of the growth of the shell does not seem to 

 have been related by any writer. Johnston says (Proc. Roy. Soc. 

 Tasmania, 1878, p. 24) : " In the young state the shell is simple 

 and resembles the common Ancylus in the same neighbourhood." 



From the fact that Ancylus woodsii (op. cit. p. 23) is omitted 

 by its author from his last catalogue (op. cit. 1890, p. 145) I infer 

 that he now considers that name to be a synonym, and further 

 that he considers it a synonym of G. petterdi. If so, it is a matter 

 for regret that Mr. Johnston has withdrawn his species in a 

 manner to confuse a student of his writings. 



The published figures of the juvenile shell only represent the 

 stage at which the septum is completed and the secondary growth 

 is about to occur. Thanks to a series of specimens collected by 



