DESCRIPTION OF THE SPECIES OF PTERYGOTUS 237 
or hinder side, with the large basal portion almost orbicular, but 
narrowing into the neck, and the terminal lobe again somewhat 
broadened and subrhomboidal, with the serrate border placed obliquely 
to the general direction of the entire joint. 
The teeth are twelve in number, gradually longer from behind 
forwards, the first often five lines long, and three broad at base. 
They are conical, a little curved and grooved down the middle, but 
not at all articulated to the base. Being set on a straight margin 
and regularly increasing in size forwards, they present a serrate 
edge along the points, rounded off a little behind into a very small 
basal lobe 4. There are the same number of teeth in young as in 
mature specimens. 
Large tubercular plicae cover the whole surface, except the inner 
thin margin of the terminal lobe, and these are concave everywhere 
to the border, except towards the inferior angle behind the attach- 
ment of the foot (Plate VII. [Plate 18] fig. 2 c), where they present 
their convex side to the margin ; and hence this portion is easy to 
identify, even in fragments. 
The limb beyond the large basal joint is very suddenly contracted. 
The point of attachment for these is very conspicuous, as a deep 
notch in the foliaceous base, about half way up on the outer side, 
and from this point to the upper angle of the serrate tip is the line 
of greatest convexity. The basal edge is rounded below, but angular 
where it joins the inner margin. 
The other joints of the limb are represented in Plate VI. [Plate 
17] figs. 1,2; and we are able to give a more complete account of 
them in this than in any other species; fig. 1 represents the upper 
and fig. 2 the lower surface of the limb. 
b. The second joint (dasignathite) is transverse, more than twice 
as wide as long, and furnished anteriorly with a large rounded boss 
occupying the exterior third of the joint (see fig. 2), broken off from 
the limb and left attached to the coxal segment in fig. 1 at 0*. Its 
hinder articulating border is somewhat concave, its inferior angle 
rounded and produced. 
?. The following joint is subtrigonal with rounded sides on the 
upper surface, and with an obtuse angle projecting over the centre 
of the following joint, but on the under surface it is rhomboidal, 
more than twice as wide as long, and with the distal and proximal 
edges nearly parallel (¢schygnathite). 
m. The fourth joint is trigonal, the outer edges wide, convex, 
and forming the angle of the limb, terminating ina rounded process 
extending half way down the following joint. On the upper side, 
