ORTMANN: monograph of the naiades of PENNSYLVANIA. 71 



E. Coker, by whom they were collected in the Cumberland River, near Clarksvillc, 

 Montgomery Co., Tennessee. In the extreme case about half of the inner gills, 

 in their middle portion near the margins, was marsupial; in three other cases, 

 much smaller sections of the inner gills were charged, and in one case a few eggs 

 were near the central margin of the right inner gill. Along with these abnormal 

 specimens, two were received with normal marsupium, but none had all four 

 gills fully charged, as is the case in the genera Fusconaia, Quadrula etc.-" It should 

 be borne in mind that these specimens were selected and sent to me to demonstrate 

 the fact that sometimes there are eggs in the inner gills. This observation does 

 not invalidate the rule that this species normally has only the outer gills marsupial. 

 Since the genus Pleurobema undoubtedly is descended from ancestral forms, which 

 had aU four gills marsupial (as for instance Fusconaia), it is not astonishing, that 

 the present species sometimes reverts, although not fully, to this old condition 

 (Atavism). From the genus Fusconaia, to which it is most closely aUied by the 

 shell, it differs nevertheless by the fact that the placentae are always lanceolate, 

 not subcylindrical, and that their color is always white, never red. 



Glochidia (observed by myself only in the specimens just mentioned) agreeing 

 in shape and size with those of P. obliquum coccineum: 0.15 X 0.15 mm. Ac- 

 cording to Surber (1915) they measure: 0.160 X 0.175 mm. 



Breeding season: I have gravid females collected on the following dates: 

 June 3, 1911; June 20, 1911; June 21, 1911; June 24, 1909. On the first date a 

 specimen with glochidia was obtained. The species is apparently tachytictic, but 

 the duration of the breeding season is not fully known. Surber's specimens with 

 glochidia were collected on July 14. 



Remarks: A quite characteristic species, in its typical phase (caUed "pig- 

 toe" by the clam-diggers), but subject to an immense range of variation. The 

 variations concern chiefly the general shape. While the typical form (See PL VI, 

 fig. 4) is characterized by its subtriangular outline, by the rather upright shape! 

 with the gently incurved beaks not situated at the anterior end, and by the well- 

 developed radial furrow, aU these characters may vary. The commonest vari- 

 ation is that in which the posterior lower angle becomes more or less produced, 

 chiefly occurring in old shells (PL VI, fig. 8) so that the diameter becomes unusually 

 low. It is, however, observable that such specimens, when young, were quite 

 normal. In other specimens the beaks are more upright, making the whole shell 

 appear higher and shorter. The radial depression is extremely variable, often 

 different on the two sides of the same specimen, and tends in many specimens to 



'2 Lefevre & Curtis (1912, i). 120) cnumenite this species under the " Tetragena;," where the mar- 



suijiuin comprises all four gills. 



