Page:The Eurypterida of New York Volume 1.pdf/344

 inner spine of each pair is the longer, and the length increases for each successive set, from the third to the fifth joints; the spines on the sixth are short. The coxal joints are elongate, slightly curved, widest at the base and equal in length to the succeeding two joints. They increase in length with each successive pair, and bear on the inner end a series of 15 or more, sharp, curved teeth, which decrease in size from the front. On each of the fourth coxal joints is a large perforation of the upper side near the fixed end. No epicoxite has yet been observed. The second joint of the endognathite is divided by constrictions into three transverse sections. It is articulated at the fixed end of the coxal joint and is fully two thirds as wide.

The limbs of the sixth pair are narrow and paddlelike, and consist of eight joints and a rudimentary ninth or claw, the seventh and eighth forming a slightly expanded blade. The seventh carries a large, subtriangular, lobelike plate, nearly one half as long as the joint proper, marked off from the inner, distal end by a suture. The eighth, or palette, is elongate oval with the margin finely incised, and carries the minute claw inserted in a notch on the inner side near the tip. On the dorsal surface of the seventh and eighth joints, at the proximal end and outer edge of each, is a group of minute, craterlike tumescences, which were probably receptacles for the bases of hairlike bristles. In a swimming arm 29.5 mm long, exclusive of the gnathobase, the narrowest joint or the fourth, measures 5 mm across; from this point the arm gradually enlarges to 6.8 mm on the seventh and eighth joints. When the swimming arms are turned back, they reach the line of the fourth or fifth dorsal segment. The gnathobases have the form of an upright retort. The inner extremity of each is provided with from 18 to 20 sharp, slightly curved teeth, which become finer posteriorly. A gnathobase, accompanying a cephalothoracic shield 22 mm long, is 16 mm long; the width of the narrow necklike portion 6 mm; the width at the base 13 mm.

The metastoma is elongate cordate, the greatest breadth coming anterior to the middle. The smaller, or posterior end, is truncated and has rounded corners. The anterior notch is rather deep and broad. A comparison of the length to the greatest breadth of several metastomas gives the proportion of 2 : 1. In an individual having a cephalothorax 23 mm long the metastoma is 16 mm long.

The genital appendage differs noticeably in the two sexes. In the female it consists of two parts, one carried by the operculum, the other by the second sternite. The opercular appendage is a slender, sagittate-based, convex, scalelike sheath projecting for about one fifth its length beyond the posterior edge of the operculum and appearing to be formed by the fusion of two parts. The anterior of these includes the sagittate base and a narrower, more convex portion with a flattened border on