319 



any such region have yet been observed in AsteroUastus. The summit, also, 

 is very differently constructed in these two genera. In AsteroUastus there 

 is a central oral aperture, immediately surrounded by five apical plates, 

 and the ambulacral areas, which are comparatively broad and short, do 

 not reach to the centre. No traces of the oral aperture are visible any- 

 where on the summit of Astroeystites, the mouth of that genus being 

 apparently subtegminal, and the ambulacral areas, which are long and 

 narrow, extend to the centre, where their covering plates interlock. 



" The ambulacral areas of Astrocystite,s are somewhat like those of 

 Blastoidocrinus, but, in the latter genus (which is still known only from 

 the few fragments collected by E. Billings from the Chazy limestone of 

 the Island of Montreal and its immediate vicinity, and from the imper- 

 fect specimens from the " Orthoceratitenkalk " of Pulkowa, Russia, 

 described and figured by Friedrich Schmidt) the spaces between them are 

 CQmpletely filled with the large deltoids, which, according to E. Bil lings 

 " extend the whole length of the pseudambulacra." 



" There are, also, apparently, some points of resemblance between 

 Astroeystites, and CystoUastus, Volborth, but in Zittel's description of the 

 latter genus, which is the only one that the writer has access to, there 

 are said to be two pectinated rhombs in the calyx, whereas no traces of 

 such structures have been observed in the dorsal cup of Astroeystites. 



" In 1874* Schmidt expressed the opinion thaA, Blastoidocrinus, Astero- 

 hlastus, Mesites and Cystoblastus are all cystidea which may be regarded 

 as intermediate in their characters between that class and the blastoids, 

 and it is quite clear that these are the genera to which Astroeystites is 

 most closely allied. ' Blastoids,' writes Dr. Charles Eastman, in the first 

 volume of his Translation of Zittel's Text-book of Palaeontology, publish- 

 ed in 1896, ' have not been recognized, as such, up to the present time, in 

 strata lower than the Silurian ; but it is possible that several genera 

 occurring in the Ordovician of North America and Russia {Blastoido- 

 crinus, Asterohlastus, etc.), which are now referred to the Cystids, may 

 eventually be transferred to the Blastoidea.' In that event, Astrocyntites 

 would, of course have to be included in the same category. On the other 

 hand, Etheridge and Carpenter, on page 129 of their 'Catalogue of the 

 Blastoidea in the Geological Department of the British Museum,' publish- 

 ed in 1886, say distinctly 'we have no certain evidence of the existence 

 of true Blastoidea anterior to the Upper Silurian period. For we much 

 doubt, as we have explained in the previous chapter, whether the pro- 

 blematical Blastoidocrinus from the Lower Silurian of Canada and Russia 

 can properly be referred to this group." Nicholson and Lyddeker, in the 



* "Memoires de I'Academie Imperiale des Sciences de St.-Petersbourg, Vile Sirie, 

 tome XXI, p. 25." 



