203 
have the summits very oblique and straight or slightly hollowed out, they are horizontally 
striated and broadest a little above the middle, growing narrower again towards the inferior 
extremity. The alae have the summits rounded (specimen from Station 204). Parietes not 
permeated, radii without pores. Basis calcareous. 
The other specimens (from Station 289) are smaller and coloured white, or yellowish in 
consequence of adhering mud. They have the large and toothed orifice of the larger specimen, 
and the shape of radii and alae are the same. The opercular valves closely resemble those 
of the specimen from Station 204. 
The scutum (PI. XX, fig. 1, @ and c) has the occludent margin somewhat convex 
towards the apex; its tergal margin is slightly hollowed out, and its basal margin is undulating. 
The ridges of growth on the outer surface are very feeble. On the inner surface the articular 
ridge is not prominent, its margin runs parallel to the tergal margin and extends almost to the 
basi-tergal angle. There is a shallow depression for the lateral depressor muscle, and a slight 
trace of an adductor ridge parallel to the articular ridge. . 
The tergum (Pl. XX, fig. 1, 6 and @) is narrow, elongated in the direction from the 
apex towards the tip of the spur. Its carinal margin is convex, its scutal margin almost straight, 
its basal margin makes only a very feeble angle with the posterior margin of the spur. The 
outer surface shows a very shallow depression at the place of the longitudinal furrow. The 
spur is placed close to the basi-scutal angle, its freely-extending part is not very long, it is 
about one third of the width of the valve. On the inner surface the articular ridge is separated 
from the scutal margin by a broad furrow; the crests for the depressores are distinct but short. 
The epidermis covering scutum and tergum is slightly more persistent than in other 
species and bears very short yellowish hairs over the whole surface of the valves; they are 
disposed somewhat more densely along the opercular margins of these valves. 
With regard to the structure of the animal’s body the following is of interest: 
Mouth. Labrum (Pl. XX, fig. 2) has a shallow notch and four larger teeth on 
each side of it. The notch is very wide at the entrance, the margin of the lateral parts is 
horizontal and quite straight. 
Palpi are not very broad, elongately-oval. The free extremity is distinctly rounded, 
superior margin nearly straight, the inferior distinctly arched. The superior margin bears a 
continuous row of hairs, which grow longer towards the free extremity. A large part of the 
outer surface which extends along the inferior margin to half way the length of the palpus, 
is covered with rather long hairs. The hairs on the inner surface, near the superior margin, 
are reflected; most of them are short, bent downwards and feathered. 
Mandible (Pl. XX, fig. 3 and 3%): distance between extremities of teeth 1 and 2 
and that between 2 and 3 nearly equal; 2°4 and 3'4 teeth indistinctly double. Third tooth 
blunt, fourth not so strong, but with rudimentary additional tooth. The part behind the 4 
tooth not much developed: at one side the whole inferior part consists of a few, nearly 
equally strong, flat teeth, at the other side, a rudimentary 5‘ tooth and two flat spines, the 
second of which is long and sharp can be readily distinguished. 
Maxilla (PI. XX, fig. 4): somewhat elongate; a small notch beneath the upper pair 
75 
