272 THE FOSSIL ECHINOIDEA OF WESTERN SIND. 



the other two forms have this, and, moreover, the madreporite reaches back between the 

 posterior oculars I. and V. There are no sutures in Palceostoma, but they are present 

 between all the plates in the forms we ,have called Hemiaster. 



" The peristome of Palceostoma is pentagonal at all ages ; that of our Hemiaster s 

 is never pentagonal, and is of the crescentic shape of the genus, the front edge being 

 curved widely and the back lip being also curved and projecting downwards as a promi- 

 nent structure. In Palceostoma the interiadial plates / at the margin of the mouth are 

 large, nearly equal in their considerable space, and the ambulacral plates contribute but 

 slightly to the margin. In the other forms only the plate / of interradium 5 is large, 

 and, as in all Hemiasters, the other first plates barely contribute to the peristome, 

 whilst the greater part of the margin is made up of the ambulacral plates. The first 

 interradial plates of 1, 2, 3, and 4, not very wide posteriorly in Palceostoma, are decidedly 

 wide in the Hemiasters. 



" There is a most remarkable heteronomia of the interradium 1 in Palceostoma 

 mirahile, but there is nothing of the kind in the two other forms. They present the 

 normal heteronomia of the inteiTadia, actinally, which is usual to such Spatangoids. 

 There is no plastron in Palceostoma, but there is in the other forms. 



" The very broad ambulacra on either side of the odd interradium in Palceostoma 

 are nbt characteristic of the Hemiasters ; on the contrary, their ambulacra are narrow. 

 In Palceostoma there is a remarkable enlargement of the fifth ambulacral plate of amb. I. 

 row a, and it is pushed to the left because plate bS oi interradium 5 is placed so much 

 to the front of a ^ of the same interradium that there is a vacant space. This is not 

 the case in the two species which we have named Hemiaster, and plates a 3 and h 3 oi 

 the odd interradium are as is usual in the Spatangoids. 



" We consider that we have proved that the generic characters of Palceostoma are 

 not present in the form we named Hemiaster elongatus, nor are they in the closely allied 

 species H. digonus, d'Archiac." 



Since this reply was published. Professor Loven kindly assents to our original 

 determination. But he doubts the propriety of permitting so-called Hemiasters with 

 only two generative pores and a backwardly passing madreporite to remain in the genus 

 as established by Desor. (November 1884.) 



