166 PROCEEDINGS OF THE ACADEMY OF [March, 



Well-grown specimens have a diameter of 9 to 10 mm., with about 

 5 whorls. 



Segmentina obstructa (Morel.)- 



Except for the teeth, this species is not distinguishable from Plan- 

 orbis liebmanni Dkr. It does not, however, attain quite so large a size, 

 the largest we have seen measuring slightly less than 9 mm. diameter. 

 The teeth are found in very young shells also ; but never more than one 

 set is present at any stage of growth, so far as seen. Specimens are 

 before us from the following places, all in Texas : 



Austin (E. Hall); San Marcos, Hays county; New Braunfels, Comal 

 county; Hondo river north of Hondo, Medina county (Pilsbry and 

 Ferriss) ; Brownsville, Cameron county (sent by G. H. Clapp) ; Hidalgo, 

 Hidalgo county (Singley) ; Rio San Filipe, Devil's river and Pecos 

 river, Val Verde county (Ferriss and Pilsbry). 



It has also a wide range in Mexico. 



The genus Segmentina was based upon the European species S. 

 nitida Miill. This is a very glossy, flattened shell with acutely angular 

 periphery, simple thin lip, deeply embracing whorls, and barriers com- 

 posed of three laminae (parietal, basal and upper) transverse to the 

 whorl, leaving a narrow, three-branched space between them. 



In eastern Asia a modification of this type is found in such species 

 as S. largillierti (Phil.), forming the subgenus Polypylis Pils. The 

 shell is less compressed and not carinate, but glossy with deeply 

 clasping whorls. The parietal lamina is obliquely transverse, the 

 others transverse, basal long, a shorter one in the outer wall, and one 

 or two in the upper margin. There are several or many barriers. 



The American forms, subgenus Planorbula Hald., have less smooth 

 and much less compressed shells, the whorls only slightly clasping, 

 often angular or subangular on the right side but rounded peripherally. 

 There are six laminae: a sigmoid, obliquely entering parietal with a 

 small tubercular denticle near its lower or left end ; a transverse basal ; 

 an obliquely entering outer lamina with a transverse one above it, and 

 a small entering lamina in the upper margin. 



In S. armigera the entering lamella in the outer margin curves up- 

 ward slightly at its inner end. In S. wheatleyi all of the laminae are 

 much more strongly developed, and the entering outer one is much 

 longer, running up in a long curve behind the transverse lamina above 

 it. The structures are, however, fundamentally identical in the two 

 species. 



In the Antillean and Mexican group to which S. obstructa belongs 



