i8i 



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 (PI. XV, fig. ijó and d) in some of the larger specimens is distinctly 

 striated longitudinally, in others no tracé 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. 



Pal pi 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 (PI. 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 rd tooth rudimentary: 4* footh represented by a triangular 

 pointed knob, 5" 1 tooth and inferior angle together represented by a blunt knob. Tooth 2 

 distinctly double-pointed, tooth 3 also doublé, but blunt. 



Maxi 11a (PI. 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 (PI. 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. 



53 



