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 Mill. This is a very glossy, flattened shell with acutely angular 
periphery, simple thin lip, deeply embracing whorls, and barriers com- 
posed of three lamin (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 JS. 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 lamine: 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 laminz 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. 
