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short and slightly prominent; the adductor ridge is more strongly developed than in Darwin’s 
figure and description. It can be traced to the apex of the valve, and it describes downwards 
a well-defined arch. The cavity for the lateral depressor muscle is rather distinct, that for the 
adductor muscle, in the larger specimens, has developed into a regular pit. 
The tergum (Pl. XV, fig. 176 and @) in some of the larger specimens is distinctly 
striated longitudinally, in others no trace of such striae is seen. The longitudinal furrow is 
well-developed in most specimens. The length of the spur is rather unequal in different 
specimens, even in those from the same locality. Where it is long, its length surpasses that of 
the part of the basal margin situated between the spur and the basi-scutal angle. The spur 
is short in the specimen from Station 273. In the same specimen, the scutal margin of the 
tergum is much less curved than is the case in the specimens from other localities. 
The structure of the parts of the mouth and of the cirri was found much as is described 
by Darwin. For the comparison of these parts with the same parts in other species, it has, 
however, been thought useful to give here a more detailed description. 
Mouth. Labrum (PI. XV, fig. 18) with the notch wide at the entrance, but not very 
deep. Lateral parts rounded, with 3 small teeth on each side of notch. Shape of subtriangular 
shield-like thickening considerably broader than its longitudinal dimension. 
Palpi broad, stout, strongly swollen in the distal part, their shape being quadrate, with 
rounded angles: the upper margin nearly straight, going over with a rounded angle into a 
distal margin, which, with a broader rounded angle, goes over into a much shorter basal margin, 
The outer surface bears numerous. spines or bristles on a triangular portion, near the angle 
formed by the upper and distal margins, the lower spines being disposed along a curved line, 
the hollow side of which is directed towards the under margin. Hairs along upper margin and 
inner side numerous. 
Mandible (Pl. XV, fig. 19) has the free edge long, in so far as the distance between 
the extremities of teeth 2 and 3 equals or even surpasses that between the extremities of teeth 
1 and 2. The part behind the 3"¢ tooth rudimentary: 4» footh represented by a triangular 
pointed knob, 5 tooth and inferior angle together represented by a blunt knob. Tooth 2 
distinctly double-pointed, tooth 3 also double, but blunt. 
Maxilla (Pl. XV, fig. 20) has a small notch behind the upper pair of spines, and has 
the inferior part of the edge developed into a very prominent step-like projection. Two long 
spines are situated on this projection, which, moreover, bears at the inferior angle a number 
of spine-like hairs. Between the notch and the step-like projection, the nearly straight edge 
bears 5—6 pairs of spines, and a single spine. Apodeme long. The step-like projection of 
one of the maxillae of the specimen from Station 273 was occupied by 3 spines, the middle 
one being somewhat thinner and shorter. 
‘Outer maxillae (Pl. XVI, fig. 1) have the outer lobe elongately oval, a large part 
of the inner surface furnished with numerous hairs; a longitudinal group or row of hairs extends 
from the outer to the inner lobe. Inner lobe relatively large, of a somewhat quadrate shape 
with rounded angles. In the specimen from Station 273, the hairs on the outer lobe are more 
delicate and shorter, and are disposed at greater distances from one another. 
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