302 John Young — On a Carboniferous Echinoderm. 



peculiar featiares lies in i-ts possessing overlapping plates, it may be 

 interesting to readers of the Magazine to know that besides the 

 other species with overlapping plates noticed in the above-mentioned 

 review, there were Echinoderms having their plates arranged on the 

 same plan as long ago as the Lower Carboniferous Limestone period. 

 Those to which I specially refer belong to the genus Arcliceocidaris. 



The remains of one or two species of this genus are not un- 

 common in the limestone shales of the Scottish Coalfield, and in 

 some localities, such as Craigenglen, near Campsie, the whole of the 

 parts composing the organism may be collected in a fine, state of 

 preservation, although disconnected. Having, for a considerable 

 number of years, paid attention to collecting the remains of Archceo- 

 cidaris, I am now in a position to show, from numerous well- 

 preserved ambulacral and inter-ambulacral plates, that their edges 

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

 normal species of Ecliini or Cidaridce, but must have overlapped or 

 imbricated each other to a certain extent — a feature easily recogniz- 

 able upon the edges of many of the plates. 



I had long ago observed the peculiar character of the bevelled 

 edges of the plates in Archceocidaris, but could never understand 

 their significance until I read the description and saw the arrange- 

 ment of the plates in Galveria hystrix. I have now no doubt that 

 those of Archceocidaris were ai'ranged on a somewhat similar plan. 

 It is not my intention to enter here upon a description of all the 

 various parts of Arcliceocidaris that have been found, but I may 

 briefly state that, besides the teeth of two very distinct species, I 

 have found nearly all the portions of the lantern or dental apparatus, 

 the various forms of plates composing the test, together with the 

 large primary and smaller secondary spines. These we find in a 

 condition in which all their articulating surfaces can be examined. 



It appears that only the larger spines and plates of Archceocidaris 

 were known to Prof. McCoy, who was the first to point out its 

 generic characteristics, but he does not seem to have noticed the thin 

 bevelled edges upon any of the inter-ambulacral plates which he 

 describes. These, as he states, are of two kinds — a pentagonal and 

 hexagonal series, which must have been arranged in the test in three 

 or more rows. In this they differ from all Mesozoic and recent 

 forms of Cidaridce, which have only two rows. The large oblong 

 pentagonal plates have two of the sides bevelled from above, the 

 opposite side being bevelled on the lower edge. I have also observed 

 in the same plates that one of the other two sides has a small groove 

 in which the edge of the next plate was received. Probably this 

 was an arrangement to retain the plates in proper position during 

 elevation and depression of the test. The ambulacral plates are 

 small, scale-like, and thinning from the middle towards their edges. 

 They are of irregular form, and vary from ± to _'_ inch in breadth. 

 In the centre of each plate there are a pair of small pores which are 

 placed in the middle of a slight oval depression. Most of them are 

 ornamented with one or two small tubercles, which probably bore 

 minute spines. Along with the ambulacral are to be found numerous 



