SCIENCE-GOSSIP. 



149 



-60. p. alphonsi, Desh. Moupin, E. Tibet. 



61. P. murata, Heude. Tchen-Keou. 



62. P. cutisculpta, Mlldff. Fud-Shien. 



63. P. invia, Heude. Tchen-Keou. 



64. P. secura, Heude. Kouang-.Si. 



65. P. multispira, Mlldff. Hoo-Nan. 



66. P. schistoplychia, Mlldff. Hoo-Nan. 



67. P. vallata, Heude. Tchen-Keou. 



VI. Section Exteroi'lax, n. sec. Type /'. 

 ijiiadrasi. Dextral. 



Habitat : Philippine Islands. 



68. P. trochospira, Mlldff. Cebu ; Siquicir. 



V. boholensis, Gude. Bohol. 



69. P. quadra^i, Mlldff. Luzon. 



70. P. polyptychia, Mlldff. Cebu. 



VH. Section Sykesia, Gude (1897), Science- 

 Gossip, N.S. iii., p. 332. AusTENiA, Gude ib., 

 p. 300, pre-occupied by Nevell (1878). Type, P. 

 clathratiila. Dextral, shell translucent, acutely keeled. 



Habitat : Southern India. Ceylon. 



71. P. retifera, Pfr. Nilgiri and Shevroi Hills, 

 So. India. 



72. P. clathratuloides, (iude. Anarmalai Hills, 

 So. India. 



73. P. clathratula, Pfr. Ceylon. 



pziteohis, Bens. 

 puteolus, Bens. 

 V. compressa, Sykes. Ceylon. 



74. P. caliginosa, Sykes. Ceylon. 



75. P. biciliata, Pfr. Ceylon. 



I strongly suspect that when the anatomy of the 

 Philippine species (Section Enteroplax) is investigated 

 the group will be found to differ so widely from 

 typical Plectopylis Xh'ax it will have to be raised to the 

 rank of a separate genus. The same may prove to be 

 the case with the section Sykesia. It is somewhat 

 difficult to hazard an opinion as to the primordial 

 form from which the present species of Plectopylis 

 have been evolved as no fossil forms are known, and 

 likewise it is almost impossible to judge as to 

 which of the known forms are the most archaic, 

 for the armatures of immature specimens, as 

 far as they have come under my observation, 

 throw no light on the subject, as they did in the case 

 of Gorilla (c.f. Science-Gossip, N.S. iii., 1896, p. 

 128) ; except in size and in the lengths of the folds, the 

 barriers of mature and immature shells of Pk-ctopylis, 

 are almost identical. There is one exception in this 

 respect, i.e., Plectopylis zvoodthorpei, in which, as I 

 pointed out {ante p. 16), the palatal folds of the 

 anterior series are only found in mature shells. It 

 may, however, be assumed that the simple armatures 

 preceded the more complicated, and on this assump- 

 tion 7'. aikatiita and its allies, with their complex 

 parietal barriers, must be regarded as the most 

 recent ; while in another direction, P. plectostoma 

 and its congeners, with their biserial palatal folds, 

 have presumably been evolved from some monoserial 

 predecessor, of which P. sowerhyi may be taken as a 

 less modified representative. 



{To he concluded.) 



