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to the basal margin. A broad longitudinal furrow separates the scutal part of the valve from 
the part along the carinal margin. On the latter part, the horizontal lines or ridges of growth 
are crossed by very distinct longitudinal ridges, the surface being divided into small squares 
each of which is distinctly excavated. The inner surface shows a short, strongly-curved articular 
ridge and a deep articular furrow; the crests for the musculi depressores are not visible, the 
valve, however, is somewhat excavated at that place. 
The structure of the animal’s body may be judged from the following details : 
Mouth. Labrum much like that of 4. g/ans, but the notch less deep and only two 
small teeth on each side of it. The rhombiform shield-like portion nearly as high as broad. The 
palpi are swollen towards the extremity, with a somewhat longer distal margin; outer and inner 
surface and upper margin covered with hairs in the same way as in the above-named species. 
The mandible (Pl. XXIV, fig. 19) has five teeth and the lower angle rudimentary. 
Teeth 2, 3 and 4 are double. As in other species of Acasta, the free margin extending from 
the 1% to the 2™¢ teeth is quite straight and that between the second and third teeth is an 
exact prolongation of that between the two foregoing teeth. The fifth tooth and the lower angle 
are quite reduced and rudimentary; the inferior part of the mandible is not so strongly elongate 
as is the case in A. conica. 
The maxilla grows broader towards the free edge and has this edge rather long 
in consequence. No notch under the upper pair of larger spines; about nine spines are seen 
between this upper and the lower pair of larger spines. Of the intermediate spines 4 are 
directed with their free extremity to one side, 5 to the other. 
ipceomier maxillae (Pl. XXV, fig 2 
long, slightly shorter, however, than in 4. glans. The hairs on the inner surface are numerous 
) have the outer lobe of an oval shape, rather 
and delicate and form a dense brush at the extremity and along the upper part of the inner margin 
of the lobe. Each hair of this brush is distinctly bent near the extremity. There is a row 
of hairs along the inner margin which stand off horizontally, and this can be followed to the 
upper margin of the inner lobe. The inner lobe has its surface, which is directed towards 
the interior, rounded, and is furnished along the interior margin with numerous hairs, which 
are directed towards the basis of the maxilla. These hairs are delicate, most of them are very 
delicately feathered. 
Cirri. First pair has very unequal rami of 8 and 18 segments; those of the shorter 
ramus are not broader than those of the longer ramus; they have the inner face rounded, 
only slightly protuberant. The hairs seen on these rounded parts and those on the extremity 
of the last segment are a little stronger than the remaining ones, but the difference is not so 
striking as in A. glans. 
The cirrus of the second pair has unequal rami of 8 to 9, and 12 segments; all the 
‘segments have the inner face somewhat rounded and furnished with dense groups of delicate 
hairs. The spine-like hairs on the extremity of the last segment of each ramus are distinctly 
stronger than the other hairs. 
The third cirrus is only slightly more slender and elongate than the second; its rami 
are nearly equal and have 11 and 12 segments respectively. Tufts of hairs on the rounded inner 
Ii! 
