20 THE AMERICAN NATURALIST. [VoL. XXXV. 
natural pair, because posterior to each there are seven mar- 
ginals and one pygal — a condition which is identical with that 
in the normal carapace (compare Fig. 1). Anterior to these 
lines, however, the right and left sides of the carapace are dif- 
ferent ; on the right side are three marginals, on the left four. 
As the left side has the normal number of marginal scutes, the 
abnormality may be described as a deficiency on the right side, 
an interpretation that is supported by the fact that the outline 
of this side recedes more 
than is usual in this 
species (compare Figs. 
i and: 2). Since the 
right side is normal from 
the fourth intermarginal 
line posteriorly, the de- 
ficiency may with fair- 
ness be said to lie in the 
region covered in this 
specimen by the first 
' three marginals. 
An examination of the 
bony plates in the mar- 
ginal series of this cara- 
pace (Fig. 3) shows a 
condition substantially 
like that just described. 
done siam du mie TA bow plates, xx ^ The first four marginal 
plates of the left side 
occupy a region which corresponds to that covered by the first 
three marginals of the right. The total number of marginal 
plates on the left side is eleven, and this side may be described 
as essentially normal, notwithstanding the fact that the first 
marginal is triangular instead of quadrilateral. The total 
number of marginals on the right side is ten, and the deficiency, 
as can be seen by a comparison with a normal specimen 
(Fig. 1), lies in the region covered by the first three plates. 
As this is the region from which a scute is absent, the con- 
clusion is warranted that, in this instance, the absence of a 
