CNEMTDACTTS GTRVANENSIS. 



157 



of its characters it approaches very closely to T. ? asperrimus, Salter, sp. ; but as 

 we have no definite proof of the existence of the remarkable asperities of the upper 

 surface visible in the latter, we prefer to retain the two forms separate — the more 

 so as we have not seen any trace of the prominent spines attached to the marginal 

 plates of the ambulacra of T. ? asperrimus." A wax squeeze from this same form 

 was sent by Dr. Bather to Schuchert, who remarks (1915, p. 186) : " This species 

 is clearly a UrastereUa, a fact which was also noted by Nicholson and Etheridge. 

 . . . In its interbrachial skeleton U. gir can crisis retains youthful generic 

 characters, seen in the well-developed interbrachial axillary ossicles [the odonto- 

 phors]." Schuchert was not quite sure whether the species, Eoactis simplex, 

 described by me (above, p. 30) was identical with the species now under descrip- 

 tion, but Eoactis simplex is quite a different form. 



100 



101 



-Ad. 



Text-fig. 100.— Cross-section through the arm of Cnemidactis girvatiensis. Ad., adambulacralia ; 



Am,., ambulacralia ; /. M., infero-marginalia ; P., paxillse. x 10. 



Text-fig. 101. — Plan of ossicles seen in side view in the interradial angle of Cnemidactis girvanensis. 



Ad., adambulacralia; /. M., infero-marginalia ; O., odontophor. x )0. 



There are about thirty specimens in Mrs. Gray's collection from Thraive Glen. 

 No other species except Stenaster obtusus, Forbes, is so abundant. It is curious to 

 note that Stenaster obtusus is very widely distributed in the Ordovician of the 

 British Isles, and a nearly related, if not identical form, S salteri, Billings, is 

 abundant in the Ordovician of N. America. No form of Cnemidactis has been 

 found in any other horizon or locality, except one specimen from the Llandeilo of 

 Dow Hills in this same district in Scotland. The presence in the Girvan " Star- 

 fish bed " of peculiar genera known at present from no other region, has been noted 

 in respect to Trilobites and Cystids. The point will be referred to again later. 

 The abundance of G. girvanensis and its excellent state of preservation allow a full 

 description of the form to be given. 



Structure of Arm, (Plate XII, figs. 1, 4; Plate XIII, fig. 2; Text-figs. 100, 

 101). — The fundamental structure is best exhibited by the cross-section (Text- 

 fig. 100). The sides of the arm are bordered by two series of ossicles — the 

 adambulacralia and the infero-marginalia. The adambulacralia are the stouter of 



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