NATURAL HISTORY SOCIETY OF GLASGOW. 229 



At Thorn, near Carluke, there is a thm stratum of limestone 

 shale, overlying the main limestone of that locality, which is 

 almost exclusively made up of the remains of Archceocidaris ; yet, 

 abundant as the organism is, no specimens have turned up showing 

 the plates or spines in position. Dr Rankin, of Carluke, has sent 

 me many fine slabs of the limestone shale from this bed, which I 

 have examined very minutely. These show how very readily the 

 organism must have fallen to pieces after death, the plates, spines, 

 and other portions of the dental apparatus being completely dis- 

 united, and now, in many instances, lie assorted in the stratum 

 according to their size and density ; this arrangement of the parts 

 having, no doubt, been produced by the action of the sea currents. 

 Some of the thin layers are seen to consist almost exclusively of 

 the smaller ambulacral plates and secondary spines, while other 

 layers are made up of only the larger plates, spines, and heavier 

 portions of the dental apparatus. 



Sections of the larger spines of Archceocidaris that I have 

 prepared for microscopic examination show that they were tubular, 

 the hollow interior, now filled with calcite, being one quarter the 

 diameter of the spine. In this other characteristic, Archceocidaris 

 agrees with the other Sea-urchins possessing overlapping plates, and 

 with the family of the Diadematid^. The intimate structure of 

 a longitudinal section of a spine consists of a series of parallel rows 

 of elongated oval cells, bounded by calcareous walls. The rows of 

 cells are about twenty in number, and are arranged on each side 

 of the tubular space in quincuncial order. A cross section shows 

 the cells within concentric lines in radiating series. They form 

 beautiful and interesting objects for microscopic investigation. 

 Other portions of the organism that I have examined are also 

 composed of somewhat similar cellular structure, this being well 

 seen in weathered portions of the test and dental apparatus. 



Only one species of Archceocidaris has as yet been described 

 from our Scottish Carboniferous limestone strata, viz., A. Urii 

 (Fleming), an interambulacral plate and lower portion of a spine 

 of which were first figured by the Rev. David Ure, in his 

 '' History of Rutherglen and East Kilbride," 1793, where he states 

 (page 318), "that good specimens of both are found at Craigen- 

 glen, Campsie." It is from the washing of the weathered shales 

 of this same locality that I have obtained my best plates and 

 other portions of the dental apparatus. I am, however, satisfied, 



VOL. II. Q 



