| OCR Text |
Show 76 PROF. F. J. BELL ON THE [Feb. 19, OREASTER OCCIDENTALIS. 0. occidentalis, Verrill, Trans. Conn. Acad. I. ii. (1867), p. 278. R=2*5 to 2*17 r. Disk not high; arms not wide, tapering pretty rapidly. Lophial line not prominent, some of the ossicles provided with short, sharp, inconspicuous spinous processes. There are some spines within the apical region. About 22 superomarginal and 20 inferomarginal plates in the largest specimen examined; the latter would not seem to be completely confined to the ventral aspect, though in the process of drying they may often be drawn thither. Both sets of plates are fairly well developed, and are richly covered with granules ; on the whole they are perhaps more indistinct than in any other species of the genus. From among the granules there stands up on a few of the plates of either series a very small and inconspicuous spinous process, and the disposition of these spines differs on different arms and on different sides of the same arm. Adambulacral spinulation diplacanthid ; about seven or eight spines ordinarily developed in the inner row ; these are not so strong as are two out of the three which are developed in the outer row, where the third, if present, seems to be always smaller than the other two. The whole of the ventral surface proper is closely covered by large and coarse granules, not a few of which become almost spinous in character ; among these only a few pedicellariae are developed. The poriferous areae are arranged in three fairly regular rows along the sides of the middle line of the arm ; the areae of tbe innermost are the smallest and those of the outermost the largest in extent; at most of the nodes formed by the reticulating dorsal ossicles a small spinous process is developed, but in the adult this is nearly always inconspicuous. The granulation on these ossicles is rather coarse, though by no means so coarse as on the ventral surface, but it always leaves bare the spinous process. The madreporite is triangularly cordiform, the apex being directed towards the apical region, just outside which it is placed. Colour in alcohol said by Verrill to be greyish brown : it has something of the same colour when dried. As has been pointed out in the introduction to this paper, this species undergoes during the later stages of its growth some very considerable changes in the characters of its spinulation ; the spines in the younger being very much better developed than in the older forms. Measurements :- R 90 115 148 r 39 53 59 Breadth of arm at base 37 46 45 Hab. Western coast of Central and Northern America. |
