236 



dark-purple ; the teeth along that margin are sharp and prominent in the lower half. The articular 

 ridge is only slightly developed, it does not terminate abruptly downwards, but it merges 

 insensibly into the tergal margin. At the place of the adductor ridge, a longitudinal crest 

 which is slightly prominent is observable, but this can be traced from the apex down to the 

 basal margin of the valve. The cavities for the musculi depressores are only faintly indicated. 



The tergum (PI. XXIV, fig. 12, 6 and <l) is elongate and slightly beaked, the spur 

 is truncated, obliquely cut off, its width not quite one-fourth of width of valve. The outer 

 surface has a well-developed longitudinal furrow, which is narrow at the apex of the valve and 

 increases in width towards the extremity of the spur. Growth-ridges distinctly developed, narrow 

 and numerous. On the inner surface the articular ridge is well-developed, while the crests for the 

 musculi depressores can hardly be made out. The same dark-purple that is seen along the 

 upper part of the occludent margin of the scutum, is observable also along the inner side 

 of the articular ridge, and along the carinal margin of the tergum. 



The study of the structure of the animal's body yielded the following results : 



Mouth. Labrum with the notch not very deep, wide at entrance. Three small teeth 

 on each side of notch. Shape of rhombiform shield-like portion much as in the other species. 



Palpi somewhat square, not strongly swollen towards the extremity, hairs less numerous 

 than in A. glans, but the general arrangement is the same. 



The mandible (PI. XXIV, fig. 13) has five teeth, and the lower angle truncated. 

 The free edge is rather elongate, the lower part narrow. The second and third teeth are 

 doublé, the free margin of the fourth is dentated, the fifth is short and pointed at the extremity. 

 The truncated lower angle bears two short spines. 



The maxi 11a has the upper and lower margins almost parallel — it does not in 

 consequence grow much broader towards the free edge. This edge is nearly straight and 

 the notch under the upper pair of larger spines is hardly to be distinguished. A doublé row 

 of five spines, all of about the same length, is arranged between the upper and under pair 

 of spin es. 



The outer maxillae (PI. XXIV, fig. 14) have the outer lobe short, much shorter 

 than in the other species of Acasta I was able to compare ; the inner lobe seems to be 

 somewhat more elongate than is the case in the other species. The hairs are distributed over 

 the inner surface of the lobes in the same way as in the other species ; those observed on 

 the line extending from the outer to the inner lobe do not form a regular doublé row, and 

 those on the inner lobe are less numerous than in A. glans on the same part. 



Cirri. First cirrus has very unequal rami of 6 to 7 and 16 segments in an older 

 specimen, and of 5 and 1 3 segments in a younger one. Those of the shorter ramus are somewhat 

 broader, yet not protuberant on the inner face. Most segments of the longer ramus are of 

 an almost quadrate shape. Spine-like hairs on the inner face of the segments of the shorter 

 ramus, numerous ones at the extremity of the last segment of that ramus, but a few only 

 at the tip of the last segment of the longer ramus. 



Second cirrus also with strongly unequal rami of 6 and 9 segments in a younger 

 specimen and 6 and 7 segments in an older one. The segments are a little longer than broad ; 



108 



