Classificatory Position of Hemiaster elongatus. 235 



form In a proper antagonism to Palceostoma, which was so 

 named in consequence of its supposed ancient and therefore 

 pentagonal shape, we now give the details in full. 



The peristome is rudely crescent-shaped and is broader 

 than long. The anterior margin is hroad and curved, with 

 the convexity forwards, whilst the posterior margin is rather 

 narrow from side to side, and is curved, but less so than the 

 anterior edge, the convexity being forwards. The anterior 

 margin is prolonged backwards, on either side, behind the line 

 of the projection of the posterior lip, and ends in a rather 

 narrow rounded angle on either side. The edges of the plates 

 forming the margin are turned forwards and backwards, 

 according to their position, and this is particularly observable 

 on the posterior lip. This lip projects downwards in the 

 median line and is at its edge lower than that of the front 

 lip and the sides of the mouth. In specimens lohich are less 

 than one half the size of the type figured, the peristome is of the 

 same shape as in the adults ; all the difference is that in the 

 smaller forms the aperture is a little more open than in the 

 others. There is not the slightest approach to an equal- or 

 unequal-sided pentagon in the shape. 



In the remarks we made on the species, at the close of the 

 descriptions of the Ranikot Echinoid fauna (p. 98), it is 

 stated that " The shape of Hemiaster elongatus, which is a very 

 common fossil, is remarkable ; and its structural details sepa- 

 rate it from others. The presence of only two generative 

 pores situated on mamelons, and the fact that the madre- 

 poric body passes backwards and separates the ocular plates, 

 make the form to look much more modern than the Num- 

 mulitic." 



On reconsidering this description we do not find that there 

 is anything material to alter. But to place our species more 

 definitely we may state that the madreporic body is continuous 

 with the small right antero-lateral generative plate (No. 2), 

 and that it is distinctly separated from the generative plates 

 on either side of it (Nos. 1 and 4) by sutures. The small left 

 antero-lateral plate (3 of Loven) is separated from the madre- 

 poric and from the left posterior lateral plate (4 of Loven) 

 by sutures. In other icords, there are sutures limiting all the 

 generative plates. Nothing can be more decided than the 

 pushing aside of the plates perforated by the ovarian pores, 

 by the suturally defined and limited madreporic body. 



The margin of the peristome is composed of ambulacral 

 plates mainly ; the interradia enter in very slightly, except in 

 the instance of the posterior or odd one (No. 5 of Loven). 

 This interradiura has a plastron, and the ambulacra are sunken 



