MUSSELS OF CUMBERLAND RIVER AND TRIBUTARIES. 53 



36. Plagiola donaci/ormis (Lea). 



Tliis dainty little species is more limited in its distribution in the Cumberland 

 than either of its two relatives. A peculiarity of the species at Ualf Pone Bar was the 

 frequent unfoldin<]; of the anterior ventral portion of the shell, the inner layer being 

 folded back against the rest, as if by some injury. The specimens found here wore 

 imusually thin-shelled and frequently had the nacre well tinged with pink. Perfect 

 specimens of this sliell are among the most attractive to be found in the L'^nionidoe, but 

 the Cumberland examples, especially those from Half Pone Bar, are badly worn at 

 the umbones, so that even small specimens have the appearance of age. This is one of 

 the smallest of the mussels — too small to be of any use for manufacturing purposes. 



37. Cy progenia irrorata (Lea). 



This species is of rather infrequent occurrence in the Cumberland. We found none 

 at all in any of the tributaries, and usually found only one or two on each bed exam- 

 ined. The species seems to inhabit rather deep water, since we never saw any crawl- 

 ing around on the shallow bars. Most of the examples are rather small, and some have 

 a shallow sulcus running over the middle of the disk from the umbonal region to the 

 post\entral margin. 



A very solid shell, but of little commercial value, as it is rather brittle and has pink 

 tips. The few shells that get into the clammers' piles are generally worked tip, 

 however. 



38. OhUquaria rcjlexa (Rafinesque). Three-homed warty-back. 



One of the most common shells of the river, and found throughout its entire extoTit. 

 Although a rather small shell, this is so thick and solid that it is used to a considerable 

 extent in the manufacture of buttons, each valve furnishing one or two small blavks. 

 The species has a long breeding season, spawning through almost the entire summer, 

 the young being extruded in white cylindrical masses. Some of these spawn masses 

 were seen lying on the gravel at Half Pone Bar June 16. Shells of females are some- 

 what fuller anteriorly than the males and can usually be distinguished after some 

 practice. The Cumberland specimens are not so beautifully rayed as those from the 

 upper Mississippi. 



39. Ptychobraachus phaseolus Hildreth. Kidney-shell. 



Scattered in the upper Cumberland from the falls down to Half Pone Bar. Although 

 this is a species of rather wide distribution, especially southward, and is by no means 

 a rare shell, it is never found in great numbers or making a large per cent in any bed. 

 The clammer rarely gets over a half dozen or dozen to the ton; the nacre is white, 

 with a soft satiny luster; the shape is nearly that of Unio gibbosus, and the species 

 would probably make a fair button shell. 



40. Plychobranchus subtantus (Say). Fluted kidney-shell. 



This species in Simpson's Synopsis is placed in the genus Medionidus. Dr. Ort- 

 mann, however, has removed it to Ptychobranchus, and, although we have seen no 

 gravid examples, we are inclined to follow him in this regard on account of the close 

 resemblance of the shell to that of P. phaseohis, differing from that species chiefly in 

 its thinner shell, greater inflation, and the presence of costre on its posterior slope. 

 On account of its small size and its thinness it has no commercial value. 



4L Dromus dromas (Lea). Dromedary mussel. 



In the main river this shell is of occasional occurrence from Mill Springs Bar, in the 

 upper river, down to Red Rock Bar, below Clarksville, Tenn. We usually obtained 

 one or two specimens at a station. The shells are rather heavy and inflated, though 

 the hump on the disk, which is characteristic of the species, is not nearly as prominent 

 as in some specimens from the Washington collection obtained by Mr. Boepple in the 

 Clinch and Holston Rivers. Some of the shells are beautifully rayed, especially 



