150 Annals of the Carnegie Museum.. 



Mollusca, Dr. Bryant Walker (1914-1916) and his view is interesting 

 for the reason that it brings out points which will be later discussed 

 in this paper. In a letter to the writer Dr. Walker mentions two 

 contradictory cases, saying: "Practically all the Naiades of the Great 

 Lakes are much smaller than the examples of the same species from 

 the inland streams tributary to the lakes. This may be the result 

 of the great difference in temperature or less abundant food-supply 

 (this also possibly the result of temperature) or the combined result 

 of both." But, as indicated, this may be due to an absolutely different 

 environment, with other unexplainable factors. Again Dr. Walker 

 says: " In the case of Quadrula elliotti, which comes from a small creek 

 in N.W. Georgia, where it grows very large, the species seems to run 

 into Quadrula atrocostata of the Coosa, which never grows as large as 

 typical elliotti.'' 



Ortmann (33, 34, 36, 38, 39) noted that the diameter of certain 

 shells increases in a down-stream direction. T.vo distinct forms repre- 

 senting what was formerly considered a single species may appear. 

 Thus we have Obovaria siibrotiinda circitliis and Obovaria subrotunda 

 lens; Pleurobema obliquum and Pleurobema coccineum; Fusconaia 

 barnesiana and vars. bigbyensis and tumescens. 



Aside from the fragmentary indirect evidence and excepting the 

 work begun by Miss March and the more general observations of 

 Walker presently to be discussed, there appears to have been no 

 systematic study of these peculiar problems, which arise in connection 

 with the development of the Naiades. The rich collection of the 

 Carnegie Museum has afforded excellent opportunity for investigating 

 the matters spoken of and at the suggestion of Dr. Arnold E. Ortmann 

 the writer has undertaken to throw what light he can upon the subject 

 with the help of the abundant material which has been placed at his 

 disposition for study. This material includes the very extensive 

 collections made by Dr. Ortmann in the rivers of western Pennsyl- 

 vania and in Lake Erie during the years 1903 to 1907 inclusive, to- 

 gether with exchanges representing the fauna of outlying territories. 

 The species employed by me are those which are most abundantly 

 represented in the Museum. I have followed the arrangement and 

 the nomenclature suggested by Sterki (50) and elaborated by Ortmann 

 (33- 35. 36). Priority is accorded in this scheme to many of the names 

 applied by Rafinesque (40) which have been revived by Frierson (13) 

 and Vanatta (53) and accepted by Ortmann. The list of species used 

 by me is here given. 



