3O2 OSBORN. [VOL. I. 



tion has never before been traced to its cause, nor have the 

 flukes in this situation been noticed and described, and hence 

 a careful account of the matter is desirable. I am indebted to 

 Mr. Chas. I. Simpson of the United States National Museum 

 for the identification of the Anodonta, and for the information 

 about Lea's work, and to Dr. W. S. Nickerson of the University 

 of Minnesota for suggestions in connection with the identification 

 of the fluke. 



I. THE EXTENT OF THE COLORATION. 



The amount of the deposited salmon-tinted material is very 

 variable indeed. I will describe an extreme maximum case at 

 the outset. The shell in this instance has an extreme length 

 of 62 mm. The entire inner surface, excepting the pallial 

 line and the muscular impressions, is salmon colored ; this 

 includes the area between the pallial line and the borders of the 

 shell, and all of the hinge area. The only points at which the 

 bluish native color of the shell can be seen are the muscular 

 impressions and the pallial line. The shell is thicker than 

 normal, and the red is shown to be an abnormally thick deposit 

 by the fact that the muscular impressions and the pallial line, 

 instead of being flush with the inner surface, are beneath the 

 general level, the pallial line forming a very conspicuous furrow, 

 as in many of the Unios with their much thicker shell. The red 

 layer, moreover, is not smooth, but minutely warty-roughened, 

 and in many instances increased by folds. Its luster is dull 

 or in places distinctly pearly, especially toward the borders of 

 the shell, i. c., in the newest portions of the deposit. 



An extreme minimum stands in marked contrast with this 

 maximum case. Such a shell (described from one actual case) 

 is 75 mm. long; its general inner surface is perfectly normally 

 bluish tinted, with mother-of-pearl luster, thin and perfectly 

 smooth. The only indication of the red coloration is a small, 

 thin, very slightly granular area of red 5 mm. across, which 

 lies directly beneath the umbo and is of the same extent and 

 position in both valves. Between these extremes of little and 

 much coloration I have found every intermediate amount. 



