242 
are about three-fifths of the width of the lateral compartment at its basis. The inner structure of 
the parietes corresponds well to Darwin's description. 
The scuta (Pl. XXV, fig. 5a, fig. 6a) are elongate, narrower and thicker than in 
several other species of the genus. They are distinctly convex, the outer surface being hollow: 
bowed from the basal margin to the apex. The surface is strongly striated with longitudinal 
lines, the transverse growth-ridges are also prominent, the occludent margin is distinctly toothed 
in consequence. The longitudinal striae are much more prominent in younger (smaller) specimens 
than in larger ones. The part of the valve along the tergal margin makes an angle with the 
remaining part; it is obliquely truncated at the basi-tergal angle. The articular ridge is not 
very strongly developed, in older specimens, however, relatively more prominent than in smaller 
ones; the adductor ridge cannot be distinguished, but the cavity for the lateral depressor muscle 
is — in older specimens — well-developed. 
The terga (Pl. XXV, fig. 54, 64) are flat and in younger specimens narrow: they 
seem to grow broader with age. The part corresponding to the spur is about half the 
width of the whole valve, it is obliquely truncated, its basal edge being parallel to the basal 
margin of the valve. The scutal margin is distinctly hollowed out, in younger specimens, however, 
the apex is more strongly produced than in older ones. The growth-ridges of the outer surface 
are only feebly developed. The articular ridge is slightly developed in the younger specimen, 
in full-grown specimens, however, it is somewhat more prominent. The crests for the depressor 
muscles can hardly be made out. The spur is confluent with the basi-scutal angle in the smaller 
specimens, in the larger and older ones, the basi-scutal angle is separated from the anterior 
margin of the spur. 
The description Darwin gives of the structure of the animal's body of the present 
species, is not very extensive. I therefore insert here the following details for comparison with 
other species. 
Mouth. Labrum (Pl. XXV, fig. 7) with a not very wide, rather deep, triangular 
notch, and two or three teeth on each side of notch. When only two are present, the 
one nearer the notch may be stronger and bifid. The margins of the notch are furnished 
with delicate hairs or ciliae. The thickened chitinous portion that overhangs the oesophagus 
and represents the true labrum is quite rhombiform in this species. Its height nearly equals 
its breadth. Palpi strong, broad, of almost quadrate shape, only slightly swollen towards 
extremity and with the angles rounded off. When in rest the tips nearly touch each 
other in the median line. Outer surface furnished with a group of long delicate hairs, 
on distal part; hairs more dense towards the extremity, and reaching not quite half the 
length of the palpus. Upper margin with a row of shorter hairs, inner surface furnished with 
curved spine-like hairs which, when the palpus is not uplifted, partly overhang the crest of 
the labrum. 
The mandible (Pl. XXV, fig. 8) has the lower part elongate and the inferior 
angle pointed. The 4 and 5 teeth are broad at the basis and somewhat rudimentary, the 
third is well-developed and double, the second also double and sharply pointed. In one of the 
specimens, the outer margin of the 4" tooth is dentated as in A. contca — but it is not so 
1I4 
