500 T. H. Withers—Pollicipes levis, Sby. 
mainly on that valve seen in the young individual (Fig. 4) and on 
a much larger scutum of another individual (Figs. 5a, 6). The 
description and figure of the inner surface of the scutum is taken 
from the larger valve, now in my possession. 
Descr iption of Scutum.—General outline approaching an equate 
triangle, convex, moderately thick, divided into two unequal portions 
by a ridge extending from the apex to the basi-lateral angle, the 
occludent portion being about twice as wide as the tergo-lateral 
portion. A further ridge extends from the apex to about the middle 
of the basal margin, slightly nearer the rostral angle; basal margin 
almost straight, making with the occludent margin an angle of 
about 71°, and with the tergo-lateral margin an angle of about 
63°; occludent margin slightly convex; tergo-lateral margin pro- 
tuberant near the base, and incurved towards the apex; rostral angle 
sharp ; basi-lateral angle blunt. Surface of valve almost smooth, 
with fine longitudinal strize, which are more strongly marked in some 
specimens. The deep pit for the adductor scutorum takes up the 
whole of the lower two-thirds of the valve. From the top of this pit 
a depression runs to the apex. This depression, which is marked with 
fine oblique lines, extends on one side to the tergo-lateral margin, and 
on the other is bounded by the inner occludent margin, which is the 
same width the whole length of the valve. Length of occludent 
margin (from apex to rostral angle) 14mm., length of basal margin 
11:5 mm., length of tergo-lateral margin (from apex to basi-lateral 
angle) 15°5 mm. 
Horizon and Locality.—Albian, Gault: Folkestone, Kent. 
Comparison with other Species. — Darwin (1851, p. 67) says of 
P. unguis: ‘‘ As before remarked, this species is very closely related 
to the cretaceous P. glaber, of which it is evidently the representative 
in the Gault; the chief difference consisting in the more elongated 
form and greater size of its upper latera, which, in P. unguis, exceed 
half the length of the tergum, whereas in P. glaber they are only 
one-third of its length. The carina, in the present species, has its 
basal margin, perhaps, less pointed, and has a narrow linear channel 
along its edges; but I am not at all sure that this latter character does 
not vary. Lastly, the anterior lower latera in P. uwnguwis are thinner, 
and rather more convex, with the basal margin more arched and 
protuberant, with the external oblique ridge very much more central.” 
The scutum of P. wnguis, as expected by Darwin, is extremely like 
that of some forms of P. glaber, and apparently differs only in being 
much thicker. Some of the differences between the various valves of 
P. unguis and P. glaber given by Darwin are not apparent in some 
specimens. There is much variation in the valves of both species, and 
it is highly probable that some of the differences will eventually be 
correlated with the differences of horizon. Consequently, until a 
larger number of specimens of both species from definite horizons have 
been examined, it is not advisable to attempt to point out probable 
differences. P. wnguis is confined to the Gault, and P. glaber ranges 
right up into the Upper Senonian. 
My thanks are due to Dr. F. A. Bather, Mr. G. C. Crick, and 
Mr. C. P. Chatwin for help in connexion with this paper. 
i 
