213 



DESCRIPTIONS OF SIX NEW SPECIES OF PLEOTOPYLIS FEOM 



TONKIN. 



By G. K. GuDE, F.Z.S. 



Read Uth December, 1908. 



PLATE IX. 



A CONSIDERABLE number of shells belonging to the genus Plectopylis 

 have been kindly placed in my hands by Colonel Messager, of the 

 French Colonial Infantry, who informs me they were all collected by 

 him in different localities in the 4th Militaiy Territory of Tonkin, the 

 headquarters of which are at Lao-Kay on the Eed Kiver, on the 

 borders of Tonkin and Yunnan. Some of these shells are of special 

 interest fi'om presenting evidence of what I consider to be hybrid 

 origin. Hitherto I have found the armature of Plectopylis practically 

 constant, only a slight amount of variation within well-defined and 

 rather narrow limits having been observed ; but in examining the 

 armatures of the shells now under consideration I found that some 

 appeared to form connecting links with each other ; and this fact at 

 first considerably puzzled me, until at length I was driven to the 

 conclusion — the idea had long been present in my mind in connection 

 with other genera — that some of these molluscs had been interbreeding, 

 and thus had given rise to a number of individuals presenting some 

 features common to both parents. Viewed in this light, their 

 segregation into distinct forms, by the process of elimination of the 

 hybrid forms, no longer presented any difB.culty. 



I am not aware that the subject of hybridity in molluscs has been 

 seriously discussed. Judging by analogy of what is known in other 

 animals and plants, it is not surprising that molluscs should at times 

 intercross. The orthodox view hitherto has been to unite all extreme 

 forms connected by intermediate links into one group, and to regard 

 these groups as variable species. It certainly is an easy way of 

 disposing of an intricate subject, but it appears to me to be more 

 philosophic to look on these extreme forms as closely allied but 

 distinct species, which in some cases happen to meet on common 

 ground, and thus give rise to progeny deiiving their characters 

 through divergent lines of descent. 



In adopting this view I do not think we meet with any insuperable 

 difficulty, for it may safely be assumed that a close relationship 

 exists between most of the known species fz'om Tonkin, since they 

 nearly all have one feature in common, i.e. the posterior parietal 

 plate, which is always in these species curved and obliquely descending 

 backwards, giving off a short posterior support from the upper 

 extremity, and these features are constant, however much the other 

 structures anterior to the plate in question, as well as the palatal 

 barriers, may vary. This constancy extends to most of the known 

 species from China, and it is only with the Burmese and Indian forms 

 that the posterior parietal plate is of different character, and gives off 

 horizontal plates and other structures. 



