12 THE NAUTILUS. 



corded from the Holston river, does not appear in the exten- 

 sive collections of Mrs. Andrews and Professor Wetherby. 

 Aberrant specimens of virgata agree with the description ex- 

 cept in the matter of size. It may be suspected that littorina 

 is a form of subglobosa varying in a similar manner. 



The entire prccrosa group is something of a confusion. 

 Particularly in the smaller rivers and creeks of central Ten- 

 nessee does it take peculiar aspects which may or may not 

 deserve differentiation from the parent stock. In the Elk 

 river are forms ranging from undeniable prccrosa to subglo- 

 bosa. The same thing is true of the Duck river. A. pinguis 

 is in the Caney Fork of the Cumberland river, but typical 

 prccrosa is there also, and I have not had means of learning 

 whether pinguis is a true local race or represents specimens 

 selected from sendings of species previously named. The 

 suspicion holds good against troostiana and lewisii. The only 

 subglobosa outside of southeastern Virginia and eastern Ten- 

 nessee that may not be challenged as variants of prarosa 

 comes from Lookout creek, a tributary of the Tennessee river 

 in northern Alabama. There is still a great deal to be learned 

 about the forms of middle Tennessee. 



THE HELICOID GROUP DISCULELLA PILSBRY. 



BY T. D. A. COCKERELL. 



Lowe gave the name Placentula to a small group of Ma- 

 deirau Helices, typified by H. maderensis Wood. This shell, 

 in general form and coloration, resembles the H. polymorpha 

 group (Discida Lowe), but is easily distinguished by the lack 

 of the surface sculpture of elongate pustuliform granules, the 

 round aperture (hence the synonym H. cyclostoma Menke) 

 and strictly continuous peristome. Eight species are referred 

 to Disculella by Pilsbry, and a ninth must now be added : 



Geomitra (Disculella) cenourensis n. sp. 



Shell with max. diam. 7.2 to 9.5 mm., with the form of G. 

 dealbata Lwe., to which it is nearly related, but dark reddish- 



