RO INLAND FISHERIES. 



the adult 2t[ija. The outline is rouuded, and the umbones are 

 very prominent, and project out so as to be widely separated from 

 each other. The foot, also, is of the ploughshare shaped variety, 

 found in such clams as Yciius, Unto, and many others, and, though 

 not so represented in the figure, may be seen through the delicate 

 semi-transparent shell to extend over the entire ventral surface of 

 the visceral mass. In this it is very unlike the hatchet-like foot 

 of the adult 3Iya, which is . relatively small and projects forward 

 from the anterior surface of the visceral mass. The siphons (8), 

 however, are similar to those in the adult form, but are excessively 

 delicate and filmy, occupying so little space when retracted that 

 the shell does not gape posteriorly to accommodate them. They 

 are protracted and retracted with the utmost facility and rapid- 



ity. 



It was not difficult, however, to determine that these individuals 

 Avere young long necked clams. When arranged in a series from 

 smaller to larger forms, very slight differences between contiguous 

 individuals, as regards the outline of the shell, lead from the 

 rounded form with prominent umbones to the elongated shell of 

 the adult, in which the umbones are inconspicuous. This com- 

 parison is illustrated in Figure 1. The outlines of the shells of a 

 few individuals have been selected from a much gi'eater series. 

 They represent forms from ^^y of a millimeter to 7^ millimeters 

 (less than /^ of an inch) in length. The largest shell differs from 

 that in the adult in having the still conspicuous umbones placed 

 anterior to the middle of the shell, but the general appearance is 

 much the same, and the changes in outline from the one to the 

 other are easily followed in intermediate sizes. 



In drawing a great many outlines with a camera, two individuals 

 of the same length very frequently presented differences in out- 

 line which Avere considerable. Everyone has probably noticed 

 how great are these individual variations in the shells of the adult 

 clams, even in those cases where the shells have not become dis- 

 torted in growth by coming in contact with unyielding bodies, 

 such as imbedded stones. The outlines selected and reproduced 



