DIADEMA SETOSUM. 275 



Pacific and Eastern seas cannot be maintained, with the exception of Diadema 

 mexicanum. I add for the sake of comparison the measurements of the only 

 two species of Diadema which seem to me to be based upon permanent 

 characteristic differences. 



The height is extremely variable in all species of Diadema, the abactinal 

 system being frequently sunken considerably below the level of the abactinal 

 parts of the ambulacra, which rise like prominent ridges above the depressed 

 median interambulacral space. In the specimens of Diadema mexicanum I 

 have thus far examined this is rarely the case, the abactinal region is but little 

 sunken. The connecting ridges of the auricles are prominent, the auricles 

 themselves triangular, festooned, but less powerful than in the Mexican species. 



The spines, as a usual thing, are more slender and the verticillations closer 

 than in the Mexican species ; the miliaries small, scattered in irregular, 

 disconnected vertical lines between the primaries, the bare space of median 

 interambulacral row generally extending to the fourth coronal plate from 

 the abactinal pole. 



The original specimen of Lamarck of Diadema turcarum in the Jardin 

 des Plantes was from Martinique. 



Littoral to 1 7 fathoms. 



In the young of both Diadema and Echinothrix the spines are propor- 

 tionally larger, and being so much less numerous, give to young Diadema- 

 tidae a peculiar facies, — E. calamaria-like {PI. II C . f. 6). We find also in 

 young Diadema characters in the actinal membrane differing from the 

 adult ; the peculiar grouping, in five separate clusters,* of the buccal am- 

 bulacra! plates which appear first, is soon lost by the encroachment of the 



* In the Preliminary Report, misled by this embryonic character of young Diadematidae, I took it for 

 granted that Echinodiadema of Verrill, in which these plates are covered with short spines, were only 

 an embryonic feature. I have since that time had the opportunity of examining the buccal membrane 

 of Centrostephanus of Triehodiadema, and find that large specimens of these two genera have identic d 

 plates, covered by spines, with those of Echinodiadema, and that this character is an excellent one to 

 distinguish Centrostephanus (= Triehodiadema = Echinodiadema) from the true genus Diadema in 

 addition to its other characteristic features. 



