238 Prof. P. M. Duncan ayid Mr. W. P. Sladen on the 



with Hemiaster elongatus is the presence of two large genera- 

 tive pores, each placed upon a conical process ; the apical 

 system is, moreover, small. 



The madreporic plate is continuous loitli the generative No. 2, 

 and is placed heticeen the two large i^ored plates Nos. 1 and 4, 

 which it separates widely, and from which it is partitioned off 

 Ity distinct sutures. The madreporic also extends behind 

 those generative plates, and separates the posterior oculars I. 

 and V. nearly as much as it does the generative plates I and 

 4. It extends backwards behind the line of the ocular pores 

 and ends in a peak-like process, which slightly separates the 

 long pair of interradial plates of the odd interradium. 



The generative plate 3 is very small and broader than it is 

 long ; it is separated from the madreporic and from the gene- 

 rative plate No. 4 by sutures. There is no fifth plate. Both 

 of the conical and perforated plates 1 and 4 are limited by 

 sutures, and the water-pores are upon the madreporic plate 

 only. The ocular plates are rather large. 



On the actinal surface the peristome is seen far in front and 

 to be crescent-shaped] it is broader than long, has a curved 

 front lip and a downward-projecting curved posterior lip. 

 The plate / of interradium 5 is broad at the margin of the 

 mouth ; but the corresponding plates of the other interradia 

 are very narrow there. As is invariable in more or less adult 

 Hemiasters, the ambulacral plates contribute to the peristomial 

 margin more than the interradials. Unfortunately the pecu- 

 liar saccharine-looking fossilization prevents the sutures of 

 the plates of interradium 1 being seen ; but in one specimen 

 it is clear that the first plate is large and of the same shape as 

 in //. elongatus ; moreover, what can be seen of the following 

 plates indicates that there are two, and not only one behind 

 the first. There is no unusual heteronomia. 

 There is a distinct plastron. 



V. 



It is evident from what has been stated regarding the essen- 

 tial and characteristic structures of the three forms, Hemiaster 

 elongatus, H. digonus, and Palceostoma mirabile, that the first 

 two are closely allied, there being only some specific distinc- 

 tions between them, such as the shape of the test, the breadth 

 of the odd ambulacrum, and the greater length of the gene- 

 rative plate 3 in the first-named Hemiaster. It is also evi- 

 dent that these two species can be separated in almost every 

 essential structural detail from the species of Palaiostoma. 



Having put forward the several points which characterize 



