336 NEW YORK STATE MUSEUM 
third 33 mm and 26 mm, probably varying somewhat according to the 
amount of compression and also somewhat with the animal. From this 
point the abdomen tapers very gradually to the telson; it is divided into 
anterior and posterior parts, easily distinguished by their structure. The 
preabdomen consists of six dorsal and five ventral, transverse plates; the 
postabdomen of six annulate segments and one spiniform. 
Preabdomen. ‘The first tergal plate of the preabdomen is very narrow 
and is overlapped by the posterior margin of the shield. Its posterior 
edge is slightly convex, and its ends are rounded. The second segment .is 
twice as long as the first, its posterior edge is slightly concave along the 
muddle portion, and the posterior angles are rounded, while the anterior 
are produced, to make up, as it were, for the rounding away of the preced- 
ing tergite. ‘The succeeding tergites are very nearly equal in length, the 
fifth being perhaps a little the longest, and are about one third longer 
than the second. The posterior margins are concave as in the preceding, 
but straighter near the sides, forming almost right angles. 
The first ventral plate, or sternite (the operculum), is one third as 
long as broad, and is divided along the axial line into two equal parts. 
These are rounded off at the lateral angles, particularly the anterior, and 
excavated along the median line for the reception of the opercular appen- 
dage; the posterior edges are slightly projected on either side of this, while | 
the anterior inner angles are projected forward, forming a compound 
median lobe. The second sternite, in the female, is nearly as long as the 
operculum, and is deeply cleft for the reception of an appendage nearly 
equal to it in length. The sides are cut obliquely forward, making the 
posterior angles rather acute; the anterior angle forms small lobes, and 
the middle of the anterior edge is slightly produced. In the male the last 
four sternites, and in the female the last three, do not differ materially 
from the last four and three abdominal tergites. The relative position of 
the several plates of the sternal series to those of the tergal, is as follows: 
the opercular plate begins a little farther forward than the first tergite, 
but, owing to its greater breadth, lies beneath the line of overlap of the 
first and second tergites, while the second sternite lies beneath the overlap 
of the second and third tergites, and so on, the last sternite underlying 
the overlap of the fifth and sixth tergites, thus not extending as far back 
as the posterior edge of the sixth tergite. 
Postabdomen. The first postabdominal segment consists of a tergal 
and a sternal portion united by their appressed pleural ends. The post- 
lateral angles are prolonged into short, bladelike lobes which extend along- 
side the following segment for fully half its length. The tergal portion is 
the longer, and its posterior edge forms a broad lobe; the sternal portion 
is short, and its posterior edge is straight, while its anterior edge extends 
forward to meet the last sternite of the preabdomen. The following five 
