MUSSELS OF CUMBERLAND RIVER AND TRIBUTARIES. 



57 



While at Clarksville we were informed that the superintendent of one of the smelting 

 furnaces along the river had been trying cull shells as a flux and found them satisfactory. 

 It is doubtful whether this utilization, however, will make an important market for 

 them. 



56. Pleurobema clava (Lamarck). Club-shell. 



Generally rare, and not found at all below Burnside. The shells are all badly 

 eroded and discolored; one of them is unusually elongate, and several show a rather 

 well-marked, broad and shallow furrow in front of the posterior ridge. .We have 

 usually found this species most abundant in small streams, and this may explain its 

 absence from the greater part of the Cumberland. It is a rather handsome shell but 

 too small to have any commercial value. 



57. Pleurobema crudum (Lea). 



This species does not appear to be common or widely distributed. All our examples 

 are rather small shells, somewhat resembling a much-flattened Quadrula subrotunda, 

 but with the epidermis of a brighter yellow and the rays quite distinct, well defined, 

 and broken up into blotches. 



58. Pleurobema aesopus (Green). Bullhead. 



We did not see many examples of this species in the Cumberland, but it is common 

 enough to be well known among the clammers. In the upper Mississippi it is called 

 "bullhead" or "sheepnose," and is used in button manufacture, although it is ranked 

 as a rather low-grade shell on account of its brittleness. In the Cumberland it is so 

 hard and flinty that no attempt at all is made to cut it as it breaks saws. The clammers 

 call it "clear profit" because they are "the only ones who get anything out of it." 

 A small example obtained at Half Pone bar was of a beautiful yellow color; the older 

 ones are brown. 



The systematic position of this species is in doubt. It seems to stand between 

 Quadrula and Pleurobema. Simpson « was not certain as to where to place it, having 

 seen only one example gravid, and it with the gills partly filled. At the biological 

 station at Fairport one was found with only the inner gills filled with glochidia and 

 another with all four. Sterki & has found glochidia in all four gills. Usually, how- 

 ever, only the outer gills are used as a marsupium. 



59. Quadrula tritogonia (Barnes). Buckhorn; pistol grip. 



This is the Tritogonia tuberculata of Simpson's Synopsis. At the time the Synopsis 

 was written the gravid female was not known. The shell stood pretty much by itself, 

 and Mr. Simpson, who was struck by certain peculiar features, especially the note- 

 worthy difference between the male and female shells, formed a separate genus for it. 

 Since the discovery by various students that it bears young in all four gills, there is a 

 general tendency to place it in the genus Quadrula, and Dr. Ortmann, who was the first 

 to propose the shift, suggested the name given above. The species is quite aberrant; 

 none of the other Quadrulas resemble it very closely, the nearest approach being some 

 of the elongate Quadrulas such as cylindrica, especially the rough subspecies strigillata 

 or Quadrula trapezoides from the south. The marked difference between the males 

 and females is unique among any related forms and entitles it at least to subgeneric 

 rank. 



This species is not rare in the Cumberland and was obtained in small numbers at 

 most of the stations from the falls down to Dover. Our specimens are mostly of 

 medium size and a number have the nacre rather badly stained. They exhibit 

 but little variation among themselves or from the form as generally known. The 

 nacre of all but two is white; in these two, obtained near Clarksville, it is pink. 



a Synopsis of the Naiades, Proceedings of United States National Museum, vol. xxn, p. 745 and 764. 

 6 According to Ortmann, Nautilus, vol. xxn, no. 10, Feb., 1909, p. 100. 



