1888.] NATURAL SCIENCES OF PHILADELPHIA. 317 



Ophiomyxa flaccida, LUtken. 



Oue specimen, dredged in Bailey's Bay. 



ECHINOIDEA. 



The number of species of echinoids observed by us is six, of which 

 five had already previously been ascribed to the archipelago ; Cidaris 

 tribuloides, as far as I am aware, had not hitherto been collected — 

 at any rate I have been unable to find any mention of its occurrence 

 there. One species, Mellita sexforis, we did not ourselves collect, the 

 specimens in our possession having been kindly donated to us by 

 local collectors. 



Cidaris tribuloides, Bl. 



Fairly abundant among the coral shelters of the North Rock. 



Diadema setosa, Gray. 



This species, one of the gems among sea-urchins, is exceedingly 

 abundant in the flats about the North Rock, where, in magnificent 

 contrast to the wealth of color by which it is surrounded, its ebony- 

 black masses stand out in prominent relief from the coral shelters 

 which it inhabits. All the individuals occupied recesses in the coral 

 growth, which they had by some means probably managed to keep 

 open. It seems hardly likely that they should have crept into these 

 shelters after they had been already formed, and that the association 

 is one of mere selection. It is a noteworthy fact that while most of 

 the animal forms inhabiting this portion of the growing reef were 

 brilliantly colored, harmonizing with, and shielding, one another by 

 their party tints of red, yellow, purple, and green, these urchins were 

 alone conspicuous by the absence of any such protective cloak ; but 

 just in their case no protective guise in the form of coloring would 

 be needed, inasmuch as these animals are abundantly able to shield 

 themselves by means of their extremely attenuated spines. 



This species is also abundant in the moderately deep water that 

 lies within the reef border. 



Hipponoe esculenta, Leske. 



North Rock, and the deeper water within the growing reef. 

 Echinometra subangularis, Lcske. 



Several specimens from the flats about the North Rock. There 

 is a certain amount of variation in the coloration of the spines, 

 which ranges from olive or sea-green to purple. 



