72 HELICID^E. 



Fauna of Southern India, ii, 1868, p. 9 et seq.). Nevill, who 

 examined these fossils, however, was of opinion that their appear- 

 ance did not warrant this theory (J. A. S. B. 1, 1881, p. 128). 



It is likewise almost impossible to judge as to which of the 

 known forms are the most archaic, since the armature of immature 

 specimens, so far as they have come under my observation, throw 

 no light on the subject, as they do in the case of Gorilla ; except 

 in size and in the length of the folds, the barriers of mature and 

 immature shells are almost identical. There is one exception in 

 this respect, i. e. Plectopylis ivoodthorpei, in which the palatal folds 

 of the anterior series are only found in mature specimens. It 

 may, however, be assumed that the simple armatures preceded the 

 more complicated structures, and on this assumption P. bensoni 

 and its allies, with their complex parietal barriers, must be 

 regarded as the most recent ; while in another direction, P. plecto- 

 stoma and its congeners, with their biserial palatal folds, have 

 presumably been evolved from some rnonoserial predecessor, of 

 which P. sowerbyi may be taken as a less modified representation. 



The distribution of the genus is somewhat peculiar. The 

 centre of distribution appears to be Lower Burma, especially 

 Pegu and Tenasserim ; while no species occur to the south-east, 

 the whole of Siam and Cochin China being blanks. Going east 

 the Burmese Shan States and Laos each possess one species, 

 while Tonkin has a considerable number. Upper Burma con- 

 tributes one species from the Bhamo district P. andersoni, one 

 from Munipur P. munipurensis, and three species in the south 

 perarcta, ponsonbyi, and woodthorpei. Assam has fourteen species. 

 Going west we find another blank tract until we reach Sikkim, 

 the western limit of the genus, where there are five species. 

 Eastern Thibet provides a single form, P. alphonsi, while Southern 

 and Central China, including Hongkong, produce no less than 

 seventeen species, and a single outlier occurs in the Loo Choo 

 Islands. 



Key to the Species. 



I. Section ENDOTHYRA, Glide. 

 (Third Section of Benson.) 



Sinistral. Umbilicus moderate. Palatal folds horizontal or oblique. 



TYPE, P. plectostoma. 



Range. Sikkim, Assam, Burma. 



A. Palatal folds in one series. 



a. Shell 14-15 mm., horizontal fold below parietal 



plate , pinacis. 



b. Shell not exceeding 1 9 mm., horizontal fold absent, sowerbyi. 



B. Palatal folds in two series. 



a. Parietal plate without denticles fidtoni. 



b. Parietal plate with one denticle posteriorly macromphalus. 



c. Parietal plate with two denticles posteriorly. 



