196 PROCIJEDINGS OF THE NATIONAL MUSEUM. vol.49. 



twisted peristome, extends over northern Mindoro, Romblon, Sibu- 

 yan, and Negros, while the other, which has a smaller, more circular 

 aperture, with flatly expanded and much thickened peristome, is 

 restricted to southern Mindoro, Ilin, Semerara, and Tablas Islands. 

 These two groups may be considered as species — the first, Schistoloma 

 (Scliistoloma) alta Sowerby, and the other Schistolma (ScJiistolma) 

 mcgregori Bartsch. 



"Wliile the members of these two species present characters on the 

 various islands that distinguish them from those Of other islands, 

 these characters represent degrees of modification rather than kind, 

 and it is possible in some instances to find some specimens on different 

 islands which appear absolutely identical, in spite of the fact that the 

 average of any one set of characters, or all the characters, of a large 

 series of specimens will give different nodes when the material from 

 different islands is compared. This expression of different averages 

 in the various island races demands some recognition, and I believe 

 that this is best met by the use of trinomial names. 



It is interesting to note that two of the islands harbor two forms. 

 Mindoro has on its northeastern coast Schistoloma alta mindoroensis 

 and Scliistoloma mcgregori wehhi on the south end, while Romblon has 

 a large race of Scliistoloma alta romhlonensis and Scliistoloma alta 

 yygmaea, a dwarf race. These may be geographically separated, but 

 no definite locality beyond "Romblon" appears on the label with the 

 specimens. 



It is equally interesting to find that in one instance inhabitants of 

 two islands are so closely allied that they may be considered identical. 

 These are Scliistoloma incgregori wchhi, which occurs on southern Min- 

 doro, and on the little island of Ilin, which is separated from Mindoro 

 by a narrow strait. 



The subgenus Hololoma, seems confined to the Calamianes group, 

 and there, as far as known, to the islands of Busuanga and Coron. 



Key to the Species and Subspecies of Schistoloma. 



Lip with slit {Schistolo7na s. a.). 

 Aperture large with the peristome broadly flaring and irregularly reflected. 



Inner peristome very thick alta. 



Inner peristome not thick. 



Inner peristome flowing over and fusing with the outer peristome. 

 Shell large, altitude more than 22.5 mm. 



Reflected lip chocolate brown mindoroensis. 



Reflected lip not chocolate brown. 

 Reflected lip flesh-colored to pale brown. 



Average altitude 24.95 mm., diameter 11.31 mm romhlonensis. 



Average altitude 23.05 mm., diameter 10.91 mm sihuyanensis. 



Shell small, altitude less than 20 mm pygmaea 



