THE APPLE-SNAILS OP SIAM. 7 



iii. Adult shell less than 40 mm. high, 

 moderately thin, without a raised and 

 thickened peristome ... ... ... P. pesmei. 



This key can be applied only to adult shells and it must be 

 remembered in using it that the shells of Pachylabra experience a 

 short period of active growth in the rainy season, in which the 

 peripheral part of their mouth is absorbed and the shell temporarily 

 assumes an immature appearance. For taxonomic purposes shells 

 are best collected towards the end of the wet season or while the 

 animal is in a comatose condition. 



Pachylabra polita (Deshayes) 



1884. Ampullaria polita, Nevill, Hand L'st Moll. Ind. Mus. II, 



p. 7 ( ? with var. compressus). 

 1906. Ampullaria polita, Dautzenberg k Fischer, Journ. de Con- 



chyl. liv. p. 426 (? with var. major). 

 1911. Pachylabra polita, Kobelt, op. cit., p. 82, pi. xxxviii, figs. 1-5. 



This is an unusually distinct and well-defined species, readily 

 distinguished from all others by its regularly ovoid outline, in which 

 (in ventral view) the projection of the mouth causes little interrup- 

 tion. The spire is of considerable relative length, occupying about 

 £ of the total height, and sharply conical in form. The shell is 

 about 1 j-l| times as high as broad. There are 6£ whorls. The 

 basal whorl of the spire is broad and swollen and the suture 

 both above and below it is deeply impressed. The body-whorl is 

 less globose than in most species. It is irregularly and narrowly 

 cordiform, with the inner outline as seen in dorsal view very oblique 

 and somewhat sinuate towards the anterior extremity, which is 

 bluntly pointed. The suture is not oblique, except, in some shells, 

 just above the body-whorl, where it is apt to change its course in such 

 a way that the depth of the penultimate whorl 'becomes considerably 

 Greater in its outer than in its inner half as seen in dorsal view. 



o 



The mouth is long and narrow and its main axis forms a very acute 

 angle with that of the shell. It is pointed above and slightly 

 introverted ; below it is narrowly rounded. The lip is sharp and the 

 callus not strongly developed. The shell is imperforate or narrowly 

 rimate and the columellar border is slightly expanded over the 



VOL. IV, NO. 1, 1920. 



