226 PROCEEDINGS OF THE 



the London Geological Magazine for this year (1873), in which a 

 figure and short description are given of a new living Echinoderm, 

 dredged during the cruise of the Porcupine in the Atlantic, off the 

 Faroe islands, and which Professor Thomson has named Calveria 

 hjstrix; its peculiar feature lying in its possessing a flexible shell 

 or test with overlapping plates. 



In a short note, in the July number of the Geological Magazine, 

 I called attention to the interesting fact that, besides the few 

 other species of Echinoderms with overlapping plates noticed in 

 the above review, there were Echinoderms, having their plates 

 arranged on the same plan, as far back in time as the Lower 

 Carboniferous limestone period. Those to which I specially 

 referred, belong to the genus Archceocidaris, one of our oldest 

 known Sea-urchins. 



I may here be allowed to state, that previous to the time when 

 my observations were recorded in the Geological Magazine, no 

 palaeontologist who has described the remains of Archceocidaris 

 had, so far as I am aware, noticed that its ambulacral and inter- 

 ambulacral plates were arranged in the test in overlapping series. 

 It is therefore with the view of more fully recording this fact 

 in the Proceedings of this Society, that I now bring this paper on 

 Archceocidaris before you this evening. 



For many years past I have carefuUy preserved all the remains 

 of Archceocidaris that came under my observation, and had long 

 ago noticed the peculiar character of the bevelled edges of both 

 the ambulacral and interambulacral plates, but I never fully 

 understood their significance until I saw the figures, and read the 

 description of the arrangement of the plates, of Calveria hysirix. I 

 have no doubt that those of Archceocidaris were arranged on a 

 somewhat similar plan ; and I am now in a position to show, from 

 numerous well-preserved plates in my collection, that their edges 

 did not abut or join square, end to end, like the plates of other 

 normal species of Echini or Cidaridse, but must have overlapped 

 or imbricated each other to a certain extent — a feature easily 

 recognizable upon the edges of many plates, especially those 

 belonging to the ambulacral series, which, from their smaller size, 

 are often found in much better preservation than the larger inter- 

 ambulacral plates. 



The interambulacral plates in Archceocidaris which bore the 

 long primary spines are of two kinds — a pentagonal and hexagonal 



