DESCRIPTION OF THE SPECIES OF PTERYGOTUS 217 
transverse lineation running into open plice on the sides (fig. 43 @) 
occupies the front margin for not quite half the segment ; a few plice 
are intermixed with the lines. 
[On the caudal joint, fig. 46, a lineation, parallel to the outer 
border (a), is distinct, but it is uncertain if this specimen be of the 
same species; it has a strong median groove down the under side, 
and is less expanded in form than P. Bankszz.] 
The swimming foot, fig. 42, has a characteristic shape, the upper 
joints (fourth and fifth) are rather narrow, and the penultimate 
(p), instead of being simply conical as in H. bilobus, is ovate (more 
so than in our figure), with the outer border especially convex. It is 
notched above to receive the fifth joint, and below divided into very 
unequal lobes. The terminal palette @ is true oval, rather blunt at its 
origin and more pointed at the extremity. It is nearly equal to the 
penultimate joint in length, but considerably narrower. 
Fig. 35 is a penultimate joint, probably of the same species. 
Antenne.—Figs. 30 and 40 are presumed to belong to this species, 
as they occur with them. They are remarkably slender and straight ; 
the base is large and broad, suddenly attenuated into the shaft, 
which is only a tenth of an inch wide, and three-quarters of an inch 
long, beset with close small teeth, and furnished with three larger 
conical ones nearly straight, the central one as long as the width of 
the shaft. In fig. 40, from the Ludlow Rock, the intermediate teeth 
are a little longer in proportion. 
The basal joints, fig. 29, are flask-shaped, much more elongate 
in the neck than those of H. dzlobus, and less swelled at the base. 
[It is possible they may not belong to the species at all, but to 
Eurypterus linearis, a species which occurs in the same beds; we 
do not yet know this joint in Eurypterus.| The base is subquadrate, 
quite rectangular on the outer upper margin :—the notch for attach- 
ment of the other joints is immediately beneath this angle,—and 
tapering into the long neck, which has a sharp ridge posteriorly, and 
together with the terminal lobe equals the length of the basal portion. 
The teeth are minute. 
The post-oral plates, figs. 31 to 34, differ a little in shape from 
figs. 41 to 44, but not specifically, unless the greater amount of 
ornament in fig. 41 may be considered sufficient to separate it. The 
shape is elongate oval, the greatest width at the upper third, the 
base (9) subtruncate, the apex with a shallow obtuse notch. At the 
lower fourth (a) is evidently a tubercle of attachment, and such as 
occurs in a more linear shape in other species, Plate II. [Plate 13] 
