Herbert, L. Hawkins — Studies on the Echinoidea, etc. 199 



tends to occur regularly ou every third plate. As the figures show 

 (PI. XIII, Figs. 1 and 2), the regularity is not perfect, but in a whole 

 column the series of tubercles has an average number one-third that 

 of the plates. There are very few " sunken tubercles" apart from 

 this series. All are identical in structure with those of the inter- 

 ambulacra. 



(c) The Interambulacra of the Adoral Surface. 



On the adoral surface, and at the ambitus, the ornament of the 

 interambulacra suddenly becomes coarser and more abundant. The 

 primary tubercles increase in size, numbers, and prominence, and 

 especially in the excavation of their areolae. Although their 

 arrangement is on the same plan as that of the tubercles of the 

 adapical surface, a series of secondaries becomes enlarged to form an 

 additional transverse row towards the adapical margin of each 

 ambital plate. This new series never reaches the size of the true 

 primaries, but reproduces all the morphological features of that series 

 on a slightly smaller scale. The normal secondaries are numerous, 

 very uneven in size, and form very regular scrobicular circles around 

 the primary areolae, though many scattered ones also occur. The 

 miliaries are few, so that much of the surface of the plates towards 

 the peristome is quite smooth. 



The "raised secondaries " maintain their numbers over the whole 

 surface, but are proportionately smaller than those of the adapical 

 region. Their "areolae" seem to be more noticeably elevated above 

 the average plate level, but their mamelons are still minute and 

 lacking in prominence. I have failed to find any " sunken tubercles " 

 below the ambitus on the interambulacra, and, in view of the 

 remarkable freedom from ornament of the plates near the peristome, 

 I feel fairly confident in stating that they do not occur there. 



(d) The Ambulacra of the Adoral Surface. 



The most striking difference between the ambulacra and inter- 

 ambulacra of the adoral surface, in the matter of ornament, lies in 

 the development of unusually numerous secondary tubercles and 

 miliaries in the former areas. Both of these sets of structures are 

 far from numerous on the adapical parts of the ambulacra, so that 

 the adoral parts of these areas are distinguished from all the rest of 

 the test-surface by their abundantly granular character. In scale 

 this ornament is perhaps a little finer than that of the adjoining 

 interambulacra. 



The "raised secondaries" are present, though far from abundant, 

 and are precisely similar to those on the adoral interambulacral 

 plates. But, in contrast with the latter areas, " sunken tubercles " 

 occur in addition. They are, however, very small, and seem to 

 belong exclusively to the regular series of the adapical surface. 

 They are restricted to the immediate neighbourhood of the "pori- 

 ferous zones", and are placed with fair regularity on the perradiad 

 side of every third peripodium. The plates on which they occur are, 

 in the majority of cases, demi-plates. I have been unable to locate 

 them with certainty on the plates at the ambitus, but these are so 



