NATURAL HISTORY SOCIETY OF GLASGOW. 231 



mentioned species, it is questionable how far the three other 

 species may hold good, as their specific characters have been 

 established upon the character of the spines. M'Coy states — " It is 

 very possible that the spines of A. vetusta may really belong 

 to the upper part of A. Urii;" also, that the spines of A. Urii 

 have about five or six longitudinal rows of short strong 

 spines, and that A. triserialis has only " three longitudinal 

 rows of spines or denticulations," while A. glahrispina has a 

 smooth spine, without denticulations. In Scottish Carboniferous 

 strata spines are not uncommon having only two rows of denticles ; 

 they occur along with spines that have three or more rows with 

 the same external form. It therefore becomes a question whether 

 these spines all belonged to distinct species, or may not have 

 varied in the number of rows of denticulations on different portions 

 of the same test, or on different individuals of the same species. 



If it can ever be satisfactorily shown that the numl^er of 

 rows of denticles did not vary in the spines of the same species, 

 then we would have a character that may yet be of use in 

 the discrimination of our specimens. If, on the other hand, 

 the spines varied, then the number of rows of denticles would 

 be a point of no value in their identification, and we should 

 then have to look- for other characters, either in the plates or 

 dental apparatus. We have already shortly pointed out the 

 characters of the two kinds of teeth met with in our strata, 

 and have only to refer to the plates. These do not seem to 

 present any very well marked features by which the different 

 species can be identified ; the only slight variations I have noticed 

 are in the number of secondary tubercles that border the plates — 

 some having nearly twice as many as others of the same size. 

 Whether specific characters can be established on the number of 

 secondary tubercules, is a point which I leave for future investiga- 

 tion. In the meantime, I hope that specimens will yet turn up 

 somewhere, that will show the relation of the various disarticu- 

 lated parts to one another more clearly than any yet found. 

 The principal point, however, to which I wished to direct your 

 attention in this paper, is the interesting fact, that Archceocidaris, 

 like the recent Calveria hystrix, is characterized by the possession 

 of a movable test, made up of a series of overlapping plates ; a 

 feature which must always remain the most distinctive of this 

 genus of Echinoderms. 



