314 THE FOSSIL ECHINOIDEA 



row are regularly uniserial. The disposition of the pores of the median series is 

 sporadic, with a slight tendency to form oblique triplets, the feature being specially 

 noticeable at the ambitus, and becomes much less regular on the upper part of the 

 series. The outer series of pores are nominally uniserial, but show slight irregularities 

 here and there, and do hot form quite as strictly regular a vertical line as the inner 

 series. The ambulacral plates are very short, their height being not more than from 

 one tenth to one twelfth of the breadth ; and each is composed of three poriferous 

 plates, the adoral one a large primary, and the other two low band-like demi plates, 

 each of these contributing a pair of pores to one of the three vertical series above 

 described. The general character and the method of the arrangement of the poriferous 

 plates accords so closely with those of H. proavia, which we have discussed at length 

 on the preceding page, that it is unnecessary to again describe them in detail. It is 

 interesting, however, to note in this species, in the neighbourhood of the ambitus of the 

 large specimen, that what might be considered as the true compound ambulacral plates, 

 from their appearance and the posture of the zigzag suture in the median radial line, are 

 in reality composed of three compound ambulacral plates, which individually simulate 

 primary poriferous plates ; these apparently true compound ambulacral plates being, in 

 other words, built up of three triplets of poriferous plates. Each of the three compound 

 ambulacral plates which simulate primaries extend up to the median suture ; but the 

 middle " compound " of the trio is the largest at the inner end, expanding in depth as 

 it approaches the median suture, and causing a corresponding diminution there of its 

 ad- and aboral companion compound plates which simulate primaries. In the large 

 specimen under notice this character may be traced throughout the greater portion of- 

 the ambulacrum, though in the upper region it is sometimes indistinct and sometimes 

 interrupted altogether, the zigzag median suture being very slightly bent and irregular. 



The primary tubercles which ornament the plates are small, subequal, imper- 

 forate, and non-crenulate ; and those of the ambulacral areas are very little smaller 

 than those of the interradia. On the interporiferous areas there may be said to be 

 four vertical rows of primary tubercles ; but these do not form horizontal rows, and only 

 the two outer rows are strictly regular ; the tubercles of the two inner rows are much 

 less numerous and alternate with one another, and are often smaller than the tubercles 

 of the outer rows. Two or three compound ambulacral plates stand between each of 

 the tubercles which form the outer vertical series ; and the inner series of tubercles 

 are almost always borne on these intermediate plates, and not upon the same plate 

 as the outer tubercle. The inner tubercles scarcely extend further than midway 

 between the ambitus and the apex, the outer series only reaching the apex and the 

 peristome. 



There are nominally two vertical series of tubercles in the poriferous zones : one, 

 which is the most regular, being situated between the outer and the median vertical 

 rows of pores ; whilst the other is in the much narrower space that intervenes, between 

 the median and the innermost vertical series of pores. Much irregularity occurs in the 

 spacing of these tubercles and in their size, and numerous small miliary tubercles are 

 interspersed. There are four ambulacral plates opposite one of the interradial plates 



