^39 



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 verv distinct longitudinal ridges, the surface being divided into smal! squares 

 each of which is distinctly excavated. The inner surface shows a short, strongly- curve d 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: 



Mout h. Labrum much like that of A. glans, 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 

 p a 1 p i 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 (PI. XXIV, fig. 19) has five teeth and the lower angle rudimentary. 

 Teeth 2, 3 and 4 are doublé. As in other species of Acasta, the free margin extending from 

 the i st to the 2 nd 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 ot the mandible is not so strongly elongate 

 as is the case in A. conica. 



The maxi 11a grows broader towards the free ed^e and has this ed^e rather lone 

 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. 



The outer maxillae (PI. XXV, fig 2) have the outer lobe of an oval shape, rather 

 long, slightly shorter, however, than in A. glans. The hairs on the inner surface are numerous 

 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 foliowed 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 strenger than the remaining ones, but the difference is not so 

 strikino- 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 1 1 and 1 2 segments respectively. Tufts of hairs on the rounded inner 



