1874.] NEW SPECIES OF HELICID.E. 611 



ridge, with slight notches both above and below. A long horizontal 

 lamella is given off from centre of the apertural ridge and extends up 

 to, but does not unite with the vertical parietal lamina ; this is simple, 

 toothed on the vertical edge, curving above slightly backwards, and 

 giving off a short horizontal lamella from the lower end towards the 

 aperture ; a thread-like free lamina, rather longer, runs parallel to it 

 below. Palatal teeth simple, 6 in number, the three lowest the long- 

 est and highest, the sixth much arched outwards. 



Major diam. - 50 inch, minor diam. - 43, alt. axis 0"19. 



Hub. Discovered by Mr. W. T. Blanford at Thayatmyo, in Pegu, 

 who has kindly allowed me to describe it. 



This well-marked species is a close link to P. perarcta ; but the 

 lowest free lamella does not extend up to the aperture to unite there 

 with the parietal ridge as in that shell. The vertical parietal lamina is 

 remarkably toothed in all the shells I have examined, and the principal 

 long horizontal lamella is unbroken throughout. 



I have before alluded to an occasional reduplication of structure 

 when describing Plectopylis serica, which reduplication seems to 

 have played an important part in the development of the different 

 species, such a change becoming at last permanently established. 

 This is apparent on an examination of the species from north to 

 south, those from Burmah showing a structure more complicated 

 and with internal barriers more solid. The Himalayan, Khasi, and 

 North-Burma forms are the simplest*, while in P. cyclaspis, kare- 

 norum, achatina, and feddeni they have assumed the most compli- 

 cated form. 



In P. feddeni the parietal barrier is evidently a combination of 

 three parallel vertical laminae, the two anterior of which are first 

 united above and below by horizontal lamellae, the enclosed area be- 

 coming eventually filled with shelly matter. At the same time, in 

 these last species the tendency that is seen in many species to obli- 

 quity in the normal horizontal parallel palatal plicae has at last pro- 

 duced, as the representative of the fourth and fifth plicae, one solid 

 and nearly vertical lamina, situated immediately in front of the inter- 

 val between the vertical parietal laminae. 



It seems difficult to account for the use of the extremely contracted 

 internal form of the last whorl, as seen so largely and intricately de- 

 veloped in this group of the Helicidae. 



When breaking up a number of shells to expose the barriers, and 

 ascertain if their characters were constant, I was greatly interested 

 to find in two instances the presence of small insects that had become 

 fixed between the sets of teeth ; it has occurred to me that this is a 

 probable solution, and perhaps one of the uses which the barriers 

 serve, and to this end have been developed. 



Insect life swarms in the forests where the shells are found ; and 

 it is quite possible that certain kinds of beetles, ants, or even leeches, 

 prey upon the mollusca, and that those possessing such bars to their 

 predatory visits, supplemented by the mucous secretion which the 



* The two small forms from South India and Ceylon assimilate to these, 

 but differ in the arrangement of the palatal plie.T. 



