230 . PROCEEDINGS -OF THE 



from specimens of the teeth that have been found, that there are 

 at least two well marked species of Archceocidaris in our beds. 

 The teeth that are generally referred to as belonging to that of 

 A. Urii are broad, slightly rounded, and have serrated points. 

 They are further marked along their whole length by a central 

 rounded keel, bounded on each side by a narrow groove; the 

 inside of the teeth are also slightly grooved. Some of these teeth 

 measure fully f inch in length, by -f^ inch in breadth, but in 

 general they are of smaller size. The other form of teeth is 

 found in the same strata as those of A. Urii, but does not seem to 

 be quite so abundant. They are narrow, acutely ridged, and 

 acutely pointed, but not serrated. Their outer surface is smooth, 

 but marked with two faint lines or grooves that run along their 

 whole length ; the inner surface, by numerous transverse scale-like 

 ridges. The largest examples seldom exceed -| inch in length, and 

 are rather less than |^ inch in breadth. I showed specimens of 

 this latter tooth to Dr Wright of Cheltenham, who is one of our 

 best authorities on fossil Echinoderms, and he was satisfied that it 

 belonged to quite a distinct species from that referred to A. Urii. 

 For this form of tooth I would propose the provisional name of 

 A. Scotica, to distinguish it from that of A. Urii. I have not yet, 

 however, been able to make out any very decided differences 

 amongst the plates and spines with which these two forms of teeth 

 are associated, although probably differences do exist. Plates, 

 doubtfully referred to the genus Palmechinus or Melonites, are 

 sometimes found in the same strata with Archceocidaris, but their 

 rarity forbids the idea that any of these teeth, which are moderately 

 numerous, could belong to either of these genera ; besides, we are 

 still ignorant of the nature of their dental apparatus. In the 

 lower limestones of the Beith district, in Ayrshire, one or two 

 examples of a spine, which I have identified as belonging to 

 A. Munsteriana (De Koninck), have been found. Tliis species is 

 distinguished from that of A. Urii by its more robust form and 

 numerous close set muricated ribs. It does not, however, occur in 

 the beds in which the above two forms of teeth are most plentiful, 

 so tliat none of them can belong to it. 



Five species of Archceocidaris liave been described from British 

 Carboniferous strata, viz. : — A. Urii (Fleming), A. Munsteriana 

 (De Koninck), A. triserialis (M'Coy), A. vctusta (Phillii^s), and 

 A. c/labrisinncc (Phillips). With the exception of the two first 



