,'-' 



76 NAIADES OF MISSOURI 



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 

 PlethobasMS ; but in shell characters there is much difference. 

 It may well be stated in general terms that Pleurobema resembles 

 Quadrula as to its nutritive soft parts and Fusconaia 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. However, 

 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. Utterbackii 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 ohliqua Lamarck, 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., 

 VIII, p. 264. 



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 (19 14, 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- 



