18 THE AMERICAN NATURALIST. [VoL. XXXV. 
which bound the periphery of the carapace, except where the 
nuchal and the pair of pygal scutes reach the edge. The bony 
plates (Fig. 1, black outlines) are also arranged in three series. 
The median one is composed of twelve plates, which, beginning 
at the anterior end, are, first, a large nuchal plate; next, eight 
neural plates; and, finally, three pygal plates, one behind the 
other. Eight pairs of costal plates abut with their central ends 
on the median series, and extend laterally well towards the 
edge of the carapace. Excepting where the nuchal plate and 
last pygal plate reach the edge, the carapace is bounded by 
eleven pairs of marginal plates. Although the scutes and bony 
plates are arranged upon similar plans, the two sets of elements 
do not coincide either in numbers or in exact positions. 
The first abnormal specimen to be described is one (Museum, 
No. 1829) from the extensive series by which this species is 
represented in the collec- 
tions of the Museum of 
Comparative Zodlogy at 
Harvard College, Cam- 
bridge. It isa male and 
was collected at Lancas- 
ter, Mass. My thanks 
are due Mr. Samuel Gar- 
man for having called my 
attention to it, as well 
as the authorities of the 
museum for their liberal- 
ity in allowing me to dis- 
sect such parts as were 
needed. The carapace, 
which was about 15 cm. 
long and 11.3 cm. broad, 
Fic. 2. — Dorsal view of abnormal carapace No. 1 (Mus. had suffered somewhat 
Ammo eee y The black lines give the out- from marginal fractures, 
rue: but in no instance had a 
whole scute or plate been lost through such injuries. 
The arrangement of the scutes is given in Fig. 2, an exam- 
ination of which shows that there are two abnormal regions : 
