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 middle portion, and the posterior angles are rounded, while the anterior are produced, to make up, as it were, for the rounding away of the preceding 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 appendage; 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 postlateral angles are prolonged into short, bladelike lobes which extend alongside 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
