HETEROCENTROTUS MAMMILLATUS. 429 



species subject to extraordinary variations in the form of the primary spines, 

 so much so that any discrimination based upon the spines alone is of no 

 specific value. The capacity of rapid repair of the spines throws consider- 

 able light upon this point, for we frequently find specimens with bat-shaped 

 swollen spines (H. mammillatus) where a spine is broken off at the base, and 

 is then replaced by a long, tapering, triangular spine, identical with those 

 thus far considered characteristic of H. trigonarius. 



The test of this species is at once recognized by the small secondary tuber- 

 cles of uniform size covering the whole abactinal part of the ambulacral 

 region ; the large primary tubercles of the ambulacra extending only two or 

 three plates above the ambitus, the transition from large primaries to small 

 secondaries being very sudden. In consequence of this structure the arcs of 

 the poriferous zone almost meet along the median line of the abactinal part 

 of the ambulacral system. The abactinal system is quite distinct, with small 

 anal system ; the tuberculation of the plates not as marked as in H. trigona- 

 rius. In small specimens the large triangular ocular plates are excluded from 

 the anal system ; but in large specimens there are frequently one or two in 

 contact at apex with the anal system, the apex becoming then truncated. 

 The primary tubercles of the ambulacral system are nearly as large as the 

 adjoining primary tubercles of the interambulacral space. The secondary 

 tubercles of the interambulacral region are of same size as the secondaries 

 of the abactinal part of the ambulacral, and cover uniformly the moderately 

 wide space left between the primaries both along the horizontal sutures and 

 the median interambulacral line. The actinostome is comparatively larger 

 than in H. trigonarius. The secondary spines of H. mammillatus form a 

 sort of loose pavement, somewhat similar to that covering the test of 

 Colobocentrotus ; they cover the whole test, are small, short, usually 

 flaring and truncated at the extremity, and give an excellent external 

 character to distinguish temporarily these two species. (See H. trigonarius, 

 p. 430.) 



The actinostome occupies the greater part of the lower surface of the test ; 

 the tubercles rapidly decrease in size towards it from the ambitus, carrying 

 much shorter spines, flattened and fan-shaped, with a rounded extremity in both 

 the species of this genus. The spines of H. mammillatus vary in shape from 

 cylindrical or bat-shaped, swollen and rounded at extremity, to long tapering 

 triangular sjnnes with edge more or less well defined. Coloration of spines 

 quite varied, either uniform ash-gray or light-brown, with white rings at the 



