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are about three-fifths of the width of the lateral compartment at its basis. The inner structure of 

 the parietes corresponds well to Darwtx's description. 



The scuta (PI. XXV, fig. 50, fig. 6 a) 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-developecl. 



The terga (PI. XXV, fig. 5^, 6ö) 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 sjaur. 



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 follovving details for comparison with 

 other species. 



Mouth. Labrum (PI. XXV, fig. 7) with a not very vvide, 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. Pal pi 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 (PI. XXV, fig. 8) has the lower part elongate and the inferior 

 angle pointed. The 4 th and 5* teeth are broad at the basis and somewhat rudimentary, the 

 third is well-developed and doublé, the second also doublé and sharply pointed. In one of the 

 specimens, the outer margin of the 4 th tooth is dentated as in A. conica — but it is not so 



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