Classijicatory Positmi of Hemiaster elongatus. 237 



HemiasteTj and are followed by two smaller plates. There 

 is no resemhlance between this arrangement and that of 

 Palwostoma. 



Interradium 1 has its plates visible on the actinal surface 

 in some specimens, and its plate 1 has a very small edge at 

 the peristome, where it forms the margin of the narrow blunt 

 angle. The space occupied by this and indeed all the inter- 

 radia at the margin of the peristome, except the interra- 

 dium 5, is much less than that presented there by the 

 ambulacral plates. Although the peristomial part of plate 7 

 is narrow, the rest of it gradually increases in width, so that 

 at the level of the third ambulacral plate of ambulacrum I. 

 it is very broad, stretching across the interradium 1, and 

 having a slightly curved posterior suture, which can be 

 traced from plate h 3 of ambulacrum I. to a 5 of amb. II. 



The plate 1 is followed by two plates : one is b 2 and the 

 other is the combined a 2 and a 3, these last forming the larger 

 plate of the series. The plate a 2 is in contact with the 

 ambulacral plates 3 and a ^, of amb. II. (one half of each) , 

 and on the opposite side it is sutured to plates 2 and 3 of 

 interradium 1. This combined plate comes in contact with 

 the ambulacral plates 4, 5, and 6 b of amb. I. 



Thereisnounionof the plates so as to form an unusual hetero- 

 nomia ; on the contrary the arrangement of the plates is exactly 

 as it is in the species of the genus Hemiaster — H. ea^pergitus^ 

 Lov^n, for instance. 



In interradium 4 there is the usual double row of plates a 2 

 and b 2 following plate /, and there is no union of any others 

 as in Palceostoma. 



So far as Hemiaster elongatus^ nobis, is concerned, it has not 

 the characteristic heteronomia of Palceostoma mirabile^ Gray. 



ly. 



Hemiaster digoniis, d'Archiac, was first fully described by 

 MM. d'Archiac and Haime in their great work ' Sur les Ani- 

 maux fossiles de ITnde,' p. 220. Subsequently we examined a 

 large collection of very indifferent specimens from the Khirthar 

 series of Sind, and we published the results in our monograph 

 * On the Fossil Echinoidea of Western Sind ' (Pal. Indica, 

 ser. xiv. fasc. iii. p. 200). We gave careful drawings of 

 as much of the tests as we could on plate xxxv. figs. 4-9. 



The species is characterized by the short, broad, posteriorly 

 elevated test, the great width of the odd ambulacrum and of 

 its groove, and the position of the apical system rather far 

 back. But the principal character which it has in common 



