28 



NATURAL HISTORY BULLETIN 



Station 58. Florida : on the Pourtales Plateau, 24° 19' N. x 



81°19'W., about 200-225 fms. 1 fine specimen. 

 Locality unknown, 1 bare test. 



Echinus gracilis 



A. Agassiz,1869. Bull. M. C. Z., 1, p. 269. 1872, Rev. Ech., pi. Via, fig. 6. 



This is one of the characteristic species of the Pourtales 

 Plateau, where, Profesor Nutting says (p. 174) it is, next to 

 Stylocidaris affinis, the most abundant species of sea-urchin. 

 He also says it is the largest species from the region, except 

 Araeosoma fenestratiim. All the specimens sent to me however 

 are small, ranging from 6 to only 24 mm. in diameter. They 

 are of great interest, nevertheless, as showing the changes which 

 take place during growth in this very handsome species. The 

 changes in color are of course the most obvious. The smallest 

 specimen is nearly all white but the genital plates and the three 

 or four uppermost interambulacral plates show sufficient green 

 (dull olive in dried specimens) to form a more or less distinct 

 star on the center of the abactinal surface. In slightly larger 

 specimens the inner ends of the uppermost ambulacral plates 

 are green and in specimens 10 mm. in diameter the green has 

 extended to the ambitus; in the interambulacra, the upper part 

 and both ends of each plate are more or less fully and deeply 

 green, while in the ambulacra, the green occupies all the inner 

 h^lf of each plate, leaving the primary tubercle and poriferous 

 area white. The ocular and genital plates are more or less 

 variegated with green but the periproctal plates as a rule re- 

 main white; occasionally they too are greenish however. This 

 is essentially the coloration of the adult, though the green may 

 in large specimens extend well down onto the actinal surface. 



There is little change in the ambulacra during growth, the 

 adult plates remaining remarkably primitive, and the plating 

 of the peristome remains about the same in the larger specimens 

 as it is in the smallest. But the relative size of the peristome 

 decreases rapidly as is usual in the genus Echinus. Thus the 

 specimen 6 mm. in diameter has a peristome 3.5 mm. across or 

 nearly .60 of the test ; one 10 mm. in diameter has the peristome 

 5 mm. or .50; one 17 mm. has the peristome 7 mm. or only a 



