228 PROCEEDINGS OF THE 



plates, which forms one of the chief points of interest in this 

 organism. The large oblong pentagonal plates have two of their 

 shorter edges bevelled from above, and were overlapped by the 

 lower edges of the hexagonal plates at this point, their edges being 

 bevelled from below. The other opposite edges of the pentagonal 

 plates are bevelled or thinned off from below, and overlapped in 

 their turn the edges of the smaller plates in the ambulacral areas. 

 I have also observed, in one or two of my best preserved pen- 

 tagonal plates, that either the lower or upper horizontal edge — 

 according to the way the position of the plate may be viewed 

 in the test — has a small groove along that side, into 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, while, at the same time, it would allow 

 of a certain amount of vertical movement along their edges, as 

 well as amongst the overlapping plates in the ambulacral areas. 



The plates of Archceocidaris are somewhat thinner than similar 

 plates in other species of Cidaridse constructed upon the normal 

 plan. This feature, when taken along with the overlapping 

 characteristics of the plates, may have given a greater flexibility 

 to the test, like that observed in Calveria hystrix. If this were the 

 case, and the plates held loosely together in the living animal, it 

 may help to explain how very seldom it is that we find a specimen 

 preserved with all the parts in position. This latter circumstance 

 is all the more to be wondered at, when we consider the abundance 

 and wide distribution of the organism throughout our limestone 

 strata. Over many of the Carboniferous old sea bottoms 

 thousands of specimens must have lived and died, yet not a 

 single example, so far as I am aware, beyond the specimen from 

 Campsie formerly mentioned, shows the arrangement and position 

 of the, various kinds of plates in the test. 



The Campsie specimen, in the Hunterian Museum of the 

 University of Glasgow, consists of a crushed test, with the spines 

 in position, lying in a piece of limestone shale. The whole 

 organism, however, is so much crushed and weathered, that 

 nothing can be made clear from it as to the arrangement of the 

 plates in the test. The long muricated spines are seen lying in 

 the shale around the test in nearly their natural position, while 

 the numerous smaller secondary spines give a brush-like aspect to 

 many of the larger plates. 



