50 Mr. E. A. Smith on the 



distribution of the Tawi-Tawi species. It will be noticed that 

 the deep water (the Sibutu passage *) which separates Sibutu 

 from the rest of the Tawi-Tawi group does not appear to have 

 been a bar in any way to, or to have influenced, the migration 

 of forms to or from the Philippines or Bongao, Tawi-Tawi, 

 and Bilatan. 



Only one form occurring at Sibutu is met with exclusively 

 in Borneo, whereas seven occur in the Philippines which are 

 not known as Bornean. 



There are only four species from Sibutu which have not as 

 yet been found in any other island of the Tawi-Tawi group, 

 whereas eleven occur in Sibutu which are also met with in 

 one or other of these islands. 



It is worthy of notice that of the four species peculiar to 

 Sibutu two, namely Helix marginata and the Corasia, are 

 most characteristic Philippine forms, and altogether unrepre- 

 sented in Borneo ; also that the remaining two, Sitala orchis 

 and Ghloritis sibutuensisy have respective Philippine represen- 

 tatives. 



Of the nineteen species found in the islands of the Tawi- 

 Tawi group east of Sibutu, no less than eleven or twelve 

 occur also in the Philippines, whereas only three are common 

 to Borneo, and these are also met with in the Philippines. 



From these facts it is therefore quite evident that the land- 

 shells of the Tawi-Tawi Islands are distinctly Philippine 

 rather than Bornean, and my colleague Dr. K. B. Sharpe 

 informs me that the same is observable with regard to the 

 avifauna. 



If we proceed beyond this fact we merely enter the region 

 of theorizing and speculation. It is natural to suppose that 

 the species common to Tawi-Tawi and ' the Philippines origi- 

 nated in the latter locality, and that in the coui'se of ages 

 (how remote we know not) they have become dispersed or 

 migrated westward. 



On the other hand, we may conjecture that certain species, 

 starting originally from Borneo, have extended to Tawi- 

 Tawi, and so on to the Philippines, 



With regard to the origin of the various genera it is hopeless 

 to speculate. How, when, and where, for instance, did 

 Cochlostyla originate ? We may theorize upon such questions 

 as these, but I am afraid our conclusions will be very far 

 from convincing. 



* The dopth of this passage is marked ou tho AdininUty charts oOO 

 fathoms ; hut Mr. Everett iulbrms mo that it has beeu ktely somided bv 

 Capt. Field, of li.M.8. 'Ejjoria,' iiud that the extreme depth did uot 

 exceed 2G7 fathoms. 



