168 NATURAL HISTORY BULLETIN 



by th.e eulargement of the larger granules that ordinarily sur- 

 round the bases of the spines, and may be due to repair of in- 

 juries. 



Inferomarginal plates have, in part, three acute marginal 

 spines; there are about ten acute spines on the under side, and 

 the intervening spinules are slender and semierect, not flattened. 

 Radii 11'"'^ and eO"^"". Marginal plates 26 on a side. Madre- 

 poric plate is partly visible. No pedicellariae. 



According to Liitken 's diagnosis the essential characters of A. 

 variabilis are as follows: 



Radii 11'"'" and 52'"™ ; ratio, 1 :4.7 ; breadth of ray, 13'"'". 



Superomarginal plates large, with large biseriate or triseriate 

 spines, often two or three to a plate. On the proximal plates of 

 the rays there are commonly three large ones. 



Inferomarginal plates are covered with somewhat large flat- 

 tened secondary spines and spinules. Adambulacral spines tri- 

 seriate; those of the inner row three, compressed; two in the 

 middle row, of which the aboral one is larger and compressed; 

 outer or third series minute. 



On plate xxiii I have figured two specimens from St. Thomas 

 I., sent by Professor Liitken, as cotypes of his species, called A. 

 variabilis. They difi:er considerably in the characters of the 

 dorsal paxillae and the marginal spines, though they agree in 

 general appearance and in most other respects. "When compared 

 with ordinary Florida specimens of A. duplicatus the rays ap- 

 pear narrower, the marginal plates higher, and their spines 

 larger, while the paxillar areas appear narrower and more sunk- 

 en below the level of the marginal plates, yet in most details of 

 structure they agree very well. 



These two specimens, although from the same place, differ wide- 

 ly in the structure of the dorsal paxillge and in some other re- 

 spects, so that they might well be considered local varieties if 

 found in different localities. They may have come from different 

 environments or different depths, not indicated on the labels. 

 These and other variations were noticed by Liitken. 



The specimen figured on pi. xxiii, figure 1, is nearly like the 

 more ordinary or normal form described above. It has radii 11°^ 

 and 50'"'". The disk and radial paxillary areas are narrower 

 than in the other. The paxillae mostly lack a central enlarged 



