MUSSELS OF CUMBERLAND RIVER AND TRIBUTARIES. 63 



tlio ventral 111:11^111, leaving thin tips. The shell is easily dlstiiiguLshcd from any 

 other species in the river except from old examples of Q. subrotunda, which are always 

 more elongate and always have yellow flesh. 



Q. ebcna would probably thrive only in the lower parts of the river, although when 

 the propagation of this species becomes feasible it may be worth trying in the upper 

 river. 



78. Quadrula tuberculata Rafinesque. Purple warty-back. 



A careful study of our material, as well as of the evidence at hand from the literature, 

 convinces us that Q. granifera and Q. tuberculata, though quite markedly di'^tiiict in 

 typical cases, are really connected by intermediate forms. In some rivers, like the 

 Tippecanoe at Belong, Ind., only strongly marked tuberculata are found. In others, 

 like the Mississippi about Fairport, Iowa, only well-marked granifera are found. 

 In such streams or portions of streams as contain both species they are indistinguish- 

 able, or so connected by intergrades that no clear line of demarcation can be drawn 

 between them. In the Cumberland, the first shells seen, in the lower part of the 

 river, were identified provisionally as granifera; as we ascended the river soijie doubts 

 as to the species began to appear, while in the upper tributaries the shells were ])retty 

 clearly identified as tuberculata. This naturally introduces the question as to influence 

 of environment on shell form, which may be touched upon briefly here. 



The most striking and essential difference between tuberculata and granifera is one 

 of degi-ee of inflation, tuberculata being a flat form and granifera much inflated. We 

 have a number of cases among the Unionidse where two otherwise similar shells are 

 distinguished by this feature; among these are: Q. plicata, inflated, Q. undulata, 

 compressed; D. dromas, inflated, D. caperatu^, compressed. From our experience 

 we are inclined to believe that one usually finds the compressed species in small 

 streams, while the more inflated forms are found in large rivers. Often when a main 

 stream has plicata, the little tributaries will have undulata, especially if they are 

 rather shallow and swift streams with gravel bottoms. The more compressed form is 

 better adapted to plow into the gravel or crawl under rocks and hold its position in a 

 swift current, where the inflated form would present too much surface to the force of 

 the water. In the softer mud and weaker current of larger streams an inflated form 

 would be advantageous, helping to buoy up the animal. 



To state the situation precisely as we have found it, if one takes one of the larger 

 rivers from source to mouth, and finds both tuberculata and granifera or plicata and 

 undulata in the stream, the compressed form is likely to be in the upper stretches of 

 the river while it Is a small swift stream, and the more inflated form farther down in 

 the main body of the river where the bottom contains more mud and the current is 

 slower. Extreme fonns of either species, so far as we know, are never found in the 

 same bed, but where both are represented the foiTus run together. 



The literature relating to granifera and tuberculata is exceedingly interesting, but 

 too long to give in detail. To understand the present status of the group, however, it 

 Ls necessary to state that Simpson in his Sj-nopsis removed these two species from the 

 (Juadrula pustulosa group, where they had been previously placed, making of them 

 tlie subgenus Rotundaria on the basis of a "well-developed sulcus on the posterior 

 rlope and remarkable beak sculpture." The beak sculpture is well marked on tuber- 

 Mlata but not so well, or almost absent, on granifera. Ortmaun, finduig only the outer 

 gills used as marsupia in tuberculata, raised Rotundaria to generic rank. We have 

 usually found only the outer gills of granifera at Fairport marsupial, altliough we have 

 a record of one example with marsupia in all four gills. 



The species does not reach a very large size in the Cumbei'land. On account of 

 iu-j purple nacre it is of no value for buttons. 



