242 the american midland naturalist 



Sources of Error. 



The Ridgeway Nomenclature was used carefully following 

 directions given in it. It may well be urged that the sense of color 

 is so varied in its development among huminity that results of 

 this kind may not have the same significance for a great number of 

 those interested in these problems. But the same criticism could 

 be applied to the ornithologist who uses the Nomenclature. The 

 writers confidence in his own observations is largely based on the 

 fact that U. S. Army : ests have shown his sense of sight to be normal 

 in every respect. 



It is also true that at times the mussels migrate from place to 



place in the same stream and from the river into the tributary. 



Where a small number of shells were used in making a comparison, 



this might have some effect on the results obtained, but as the 



evidence of most observers is that migration is comparatively 



rare among them, this can have hardly any effect on the general 



impressions this paper has hoped to convey. 



Washington and Jefferson College, 

 Washington, Pa. 



X. — Literature Cited. 



1. Crozier, W. J. "Sex Correlative Coloration in Chiton tuherculatus." 

 Amer. Nat., Vol. LIV, 1920, p. 84. 



2. Faussek, V. "Uber die Anlagung des Pigments bei Mytilus." Zeit. f. 

 wiss. Zool. Vol. 65, 1898. 



3. Grier, N. M. "On the Erosion and Thickness of Fresh Water Mussels." 

 Naut. 1920. 



4. Grier, N. M. "Sexual Dimorphism and Some of its Correlations in the 

 Shells of Certain Species of Najades." Amer. Mid. Nat., 1920, Vol. 6, p. 165. 



5. Grier, N. M. "Morphological Features of Certain Mussel Shells of L. 

 Erie compared with Corresponding Ones of the Upper Ohio Drainage. 

 Annals of Carnegie Museum, 1920. 



6. Horton, A. H., Hall, M. R., Bolster, R. H., Leighton, M. D. "Surface 

 Water Supply of the United States, 1907-08. Part III, Ohio River Basin, p. 



29, 35, 47- 



7. Israel, W. von. "Die Najaden des Weidegebietes, etc." Beilage zum 

 Nachrichblatt Deutschen Malakozoologisches Gesellschaft, N. 4, 1914. P- 4- 



8. Jennings, O. E. "A Botanical Survey of Presque Isle, Erie Co., Pa. 

 "Annals Carnegie Museum, Vol. 5, 1909. 



9. Kofoid, C. S. "The Plankton of the Illinois River, 1894-99. Bull. III. 

 State Lab. Nat. Hist. Vol. VI, no. i, 1903. Ibid. Vol. VIII, 1908. 



10. Lewis, S. J. "Quality of Water in Upper Ohio Basin and at Erie, Pa." 

 Water Supply Paper 161, U. S. G. Survey. 



11. List, Th. "Uber die Einfluss des Lichtes auf die Ablagerung von 

 Pigment." Arch. f. Entwick. Mech. Vol. 8, 1899. 



