254 | General Notes. [ April, 
halves of the disc become bent towards each other like the op- 
posite valves of a mollusc. Altogether the discovery is a most 
interesting one, showing as it does the wide range of form and 
physiological differentiation which may be exhibited by a very re- 
stricted group of simple organisms. 
' 
MARGARITANA DEHISCENS Say.—A study of the anatomical parts 
of various species of Unionidæ, leads to opinions somewhat dif- 
ferent from those set forth by Mr. Lea in the last edition of his 
invaluable Synopsis, and suggests a revival of the discussion as to 
the propriety of dividing the genus Unio, as now constituted, into 
several genera or sub-genera. The soft parts of these molluscs 
have been found to present very great differences, not only in the 
position and relation of the various organs, but also in regard to 
their degrees of development. 
The species known as the Margaritana dehiscens of Say, pre- 
sents characters that set it entirely apart from the other species of 
that group examined in connection with it. These characters will 
observations shall have been made.. It is suggested that this 
species belongs to the genus Leffodea of Rafinesque. It exhib- 
its certain characters that would seem to ally it to the genus 
Mycetopus from South America.—A. G. Weatherby. 
On TEXAN STREPTOPOMATIDÆ,—The writer has recently had 
the pleasure, through the kind services of Mr. G. W. Marmock, 
of Bexar county, Texas, of examining specimens of the exceed- 
ingly rare Melania pleuristriata of Say, which, before Mr. Mar- 
mock’s discovery, was regarded as a lost species, no types being 
in existence. 
A careful study of the linguals of this species, as well as 
various characters of the shell, and the extreme south-western 
geographical range, suggests its close alliance to the genus 
Pachychetlus Vea. 
am informed by Mr. Marmock that it inhabits the springs of 
south-western Texas, in this respect having the habit of the East 
Tennessee Gomiobasis. A species closely allied, or a very Pet 
sistent variety, for which I suggest the name, at present, of variety 
marmocki, accompanies the specimens, together with an uniden- 
tified Planorbis, a Physa and a Lymnea. 
Mr. Marmock also found, in the same region, the very rare 
Holospira roemeri Pfr., the Macroceramus pontificus Gould, and the 
Helix photus Pfr. 
The description of the Pachycheilus was published by Mr. Say 
in the New Harmony Disseminator, December 30, 1839. Descrip- 
tions may also be seen in the following works: Descr. of New 
_ Shells, 8vo, p..15, 1840; Binney’s Edition of Say, p. 140. 
For the identification of the species, and the above references 
to the description, the writer is indebted to the kindness i 
Geo. W. J 
ryon, Jr—A. G. Weatherby. 
SEE a RE eee 
