NATURAL HISTORY SOCIETY OF GLASGOW. 227 



series — as was first pointed out by Professor M'Coy, and must 

 have been arranged in the test in three or more rows. In this 

 they differ from all mesozoic and recent forms of Cidaridae, which 

 have only two rows. The pentagonal plates are somewhat oblong, 

 and seldom measure, in the largest specimens, more than f inch 

 in length in their greatest diameter; the hexagonal plates are 

 generally smaller. Both series of plates have carried a long primary 

 denticulated spine upon the central tubercule, some of which 

 measure fully 4 inches in length. The plates are also bordered by 

 a series of smaller secondary tubercules, varying in number from 

 40 to 100, on the larger plates. These, as first pointed out by 

 Mr David Eobertson, F.G.S., from specimens which he collected 

 from the limestone shales of Dunbar, have likewise small slender 

 striated spines, that seldom measure more than J inch in length. 

 These small spines are well seen in position along with the larger 

 muricated spines, in a crushed and weathered test of Archceocidaris 

 from the limestone shale of Sculliongour, Campsie. 



The ambulacral plates are small, scale-like, and thinning from 

 the middle on their under surface towards their edges. They are 

 of irregular form, and vary from J to ^2 ^^^^^ ^ diameter. Each 

 plate is perforated by a central pair of small pores, which, on the 

 upper surface, are situated in the middle of a slight oval depression. 

 Most of them are ornamented with two or three small tubercules, 

 which probably bore minute spines. Along with the perforated 

 ambulacral plates are found numerous other small plates that 

 cannot be distinguished from the former, either in size or shape. 

 These, however, contain no pores; but whether they were ar- 

 ranged alternately with the poriferous plates, or formed part of 

 the base or summit of the test, I would not venture an opinion in 

 the present disjointed condition in which the plates are met with; 

 I may here state that, with the exception of the larger spines, 

 which are only found preserved to their full length in the harder 

 shales, all the best preserved fragments of Archceocidaris I have 

 found were obtained from the washing of the weathered limestone 

 shales. By this method I have obtained nearly all the plates 

 and spines that compose the test, as well as all the portions of the 

 lantern or dental apparatus, and these in a state of preservation 

 that often permits of the examination of all their articulating 

 surfaces. 



I shall now shortly describe the overlapping character of the 



