DESCRIPTION OF THE SPECIES OF PTERYGOTUS 207 
a few points. None has been observed on the head. On the body 
joints numerous small semicircular plica: (fig. 2@)—the curve open 
forwards—cover the surface, but are less conspicuous on the hinder 
portions of each segment. On the anterior narrow segment they 
are perhaps most prominent, and on this and some other of the front 
rings a raised transverse thread-like ridge runs across at about the 
middle, or a little in advance of it, but does not limit the conspicuous 
portion of the sculpture. 
Under Side of Head, and Appendages. 
In three or four specimens, figs. I, 3, 4, 9, &c., the organs of the 
mouth, with the antenne, are 77 sz¢z, and in others the appendages 
of the head occur so grouped together as to show that they were 
associated. Fig. 3 appears to have the parts most complete. 
Antenne (fig. 6, &c..—None have been found more than two 
inches long by a quarter of an inch broad at the base of the chele, 
the free joint of which is, in antennz of this size, about six-tenths 
of an inch long. The base of the fixed finger 
is but slender, scarcely thicker than the preced- 
ing joint, and is shorter than the chele, the 
shafts of which are linear and straight. (Fig. 7 is 
broader than usual, and may indicate a difference 
of sex). The points of the chelae are sharply 
incurved ; their armature is minute, of small and Fic. 1. 
larger teeth. Endognath( Mandible), 
gts natural size? of 
There are apparently but four joints in the Plerygotus bilobus, 
p, the basal or 
antennz, the basal one small, the second, fig. 7 6 Carel corte: 
(both figs. 6 and 7 are wrongly drawn as to the c-g, palpus. 
length of the lower joints, from being so much 
broken), at least five times as long as broad, the fourth (c) is the 
fixed claw and its base, and the fifth the free claw. The articulations 
of the joints are all oblique. 
Endognaths (Mandibles, &c.)\—The under side of the carapace, of 
which fig. 4 is the cast, shows the great foliaceous bases (with serrate 
inner lobes) of the swimming feet (c). No mandibles or maxilla? 
have been discovered beneath these; but in fig. 1, at «, the long 
filiform palpi of one pair of these organs extend right and left of 
the carapace. They are more elongate than in other species. 
A small mandible, found since the plate was engraved, is here 
1 The dilated bases of the swimming feet (ectognaths) were formerly supposed to be the 
mandibles, and the produced neck-like portion to be jointed ; Quart. Journ. Geol. Soc. 1. c. 
This has been set right by Professor Huxley (see Part I.). 
