LIMITS OF HEREDITARY CONTROL 885 
on scute ‘2’ (six cases) on the left, and scute ‘57’ (four cases) 
on the right. However, it should be pointed out that there are 
but four cases of exact coincidence (one involving three specimens) 
by which we mean.double scutes falling at the same point in the 
same band in two or more individuals. 
(2). Incomplete scutes. The second type of atypical scute vari- 
ation is just the opposite to that of the preceding, in that it is prob- 
ably the product of a splitting of what was originally a single 
scute, and results in the formation of an incomplete element. 
This type appears so rarely that not very much importance can 
be attached toit. In the 516 specimens examined only three cases 
of incomplete scutes were found. These occur as follows: Speci- 
men one, a female, between scutes 52 and 54 of band 6 (the small 
plate is counted as a whole one); specimen two, a shell, between 
scutes 14 and 16 of band 8; specimen three, also a shell, between 
scutes 56 and 58 of band 5. The first and second of these are 
sketched in figs. 9 and 10, respectively. The most interesting 
point brought out is that in each case one of the scutes lying adja- 
cent to the incomplete one has but three hairs instead of the typi- 
cal four, while the small scute has in each case but a single hair 
associated with it. The presence of three hairs in an adjacent 
scute would not alone be cogent proof that the primordium of 
such a scute had split to give rise to the incomplete element, as 
will be evident when the next section of the paper is reached. 
However, all of the other relations in these specimens point toward 
a fission process. 
(3). Three-hair type of scute. The third type of atypical 
variation is perhaps the most fundamental of all, because it in- 
volves a distinct change in the morphological unit of the scute. 
In this type the bony plate underlying the scute has but three 
hairs at its posterior end instead of the customary four. It was 
first detected in a shell which had an unusually high number of 
scutes in the bands; in fact the one representing the most extreme 
case of high numbers that we have so far found. When the shell 
is first observed the attention is at once attracted by the narrow- 
ness of the scutes, and upon examining them the interesting fact 
is revealed that the majority show the three-hair type (fig. 22). 
