76 PROF. F. J. BELL ON THE [Feb. 19, 



Oreaster occidentalis. 



O. occidentalis, Verrill, Trans. Conn. Acad. I. ii. (1867), p. 278. 



R=2o 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 com- 

 pletely confined to the ventral aspect, though in the process of dry- 

 ing 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 spiuulation diplacanthid ; about seven or eight 

 spines ordinaiily 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 whicli become almost spinous in 

 character ; among these only a few pedicellariaj are developed. 



The poriferous arese are arranged in three fairly regular rows along 

 the sides of the middle line of the arm ; the areae of the 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 : — 



E 90 115 148 



r 39 53 59 



Breadth of arm at base 37 46 45 



Mah. Western coast of Central and Northern America. 



