an 
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THE NAIADES OF MISSOURi 187 
teeth well developed; umbonal cavities moderately deep; nacre 
generally whitish or red. 
MISCELLANEOUS REMARKS:—The above descriptions of the 
anatomy shows the genus to be identical to that of the genus 
Plethobasus; but in shell characters there is much difference. 
It may well be stated in general terms that Pleuwrobema resembles 
Quadrula as to its nutritive soft parts and Fusconaza as to external 
shell structures. However, the Pleurobema shell does not usually 
possess such a prominent, angular, and inflated umbonal ridge as 
that of the Fusconaia; neither does it possess the yellowish color 
of the nutritive anatomy as in the case of Fusconaia. Eowever, 
the differences among the species of Pleurobema are well marked 
ecological, as well as morphological, ones; i. e., the heavier, more 
inflated forms being more as dwellers in the large rivers, and the 
* smaller, more compressed shells being found in the creeks and 
medium sized rivers. From the two following genera Pleurobema 
may be easily distinguished by not possessing such an elongate 
and straight type of shell. 
The author of this catalogue wishes to gratefully acknowledge 
the dedication of a new species of Pleurobema to him under the 
authorship of Mr. Frierson. The description and figures of this 
new species (Pleu. Ulterbackit F.) appear here for the first time, 
and, until more data can be secured, concerning its soft parts in 
gravid condition, it is placed here tentatively at the close of the 
list of Pleruobemae. 
Pleurobema obliquum (Lamarck). 
(‘‘Pig-toe,’”’ ‘‘White Pig-toe,’’ ‘‘Ohio River Pig-toe.’’) 
Not figured. 
1819— Unio obliqua.Llamarck, An. Sans. Vert., VI, p. 72. 
1900b—Quadrula obliqua Simpson, Proc. U.S. Nat. Mus., XXII, p. 788. 
1912b—Pleurobema obliquum, (Lamarck) Ortmann, An. Car. Mus., 
WARES hoy loys 
ANIMAL CHARACTERS:—The soft part of typical obliquum 
not having been seen by the author, reference is made to the des- 
criptions of the varieties of this species—the anatomy of which 
is, of course, identical. Wilson and Clark (1914, p. 61) report this 
species as occasionally bearing ova in all four gills and that the 
conglutinates have the appearance of cucumber seeds. 
SHELL CHARACTERS :—Shell trigonal, medium in size, emargi- 
