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BULLETIN OE THE UNITED STATES FISH COMMISSION. 
SOME STRUCTURAL PECULIARITIES OF THE SMALL CLAM. 
Many of the attached forms were extremely small. Several were obtained which 
were but 0.4 mm. in length, and these the unaided eye could with great difficulty 
distinguish from fine grains of sand. A glance at fig. 2, which represents an individ- 
ual of this length, shows a creature with little resemblance to the adult Mya. The 
outline is rounded, and the umbones are very prominent, and project out so as to be 
widely separated from each other. The foot (/) is of the plowshare-shaped variety 
found in Venus , TJnio , and other clams, 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-shaped 
foot of the adult Mya, which is relatively small, and projects forward from the anterior 
surface of the visceral mass. The siphons (s), 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 rapidity. 
Pig. 1. Mya arenaria. Ten camera outlines of shells varying in length from 0.4 mm. to 7.5 mm. They are intended to 
illustrate the change from a rounded outline in smaller individuals to the elongated condition of older forms. There 
is at first a more rapid posterior, and subsequently a more rapid anterior, growth of shell, which causes the relative 
position of the umbo to shift forward, and then bach to a position midway between the two extremities of the shell. 
It was not difficult to determine that these individuals were 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 comparison is illustrated in fig. 1, in which the 
outlines of the shells of a few individuals have been selected from a large series. They 
represent forms from 0.4 to 7.5 mm. (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 one to the other are easily followed in intermediate sizes. 
