IN THE MOBPHOLOGY 01" THE CYSTIDEA.. 5 



towards sucli types as Megacystis and EcJiinosplKBra, in wliicli 

 it is difficult to trace any definite symmetry, though certain indi- 

 viduals appear to have a hexamerous base; and the peristome 

 of Echinosphcera aurantium may have two, three, or four ambu- 

 lacra! extensions, thus foreshadowing the variations oiActinometra. 

 It may be noted, however, that Hall mentions a single species of 

 Crinoid from tlie Hamilton group with a hexamerous base *. Four 

 Cystidean genera, at any rate, Caryocrinus, Corylocrinus, Hemi- 

 cosmites, and Jucflandocrimts, are typically hexamerous, a point 

 which is not without interest from its bearing on the general 

 question of the morphology and phylogeny of the Echinoderms. 

 Many pentamerous Cystids resemble the types above mentioned 

 in having a dicyclic base. Thus in EcMnoencrinus (PL I. figs. 

 3, 4) the so-called basals, plates 1-4 of Forbes's nomenclature t, 

 are really infrabasals, plate 3 being a double plate. Alternating 

 with these are the subovarian series (Nos. 5-9) or first parabasals 

 of Volborth +, which are the true basals ; while the second para- 

 basals or centrolaterals, Forbes (Nos. 10-14), are, I believe, the 

 radials. The lower part of the body is constructed upon this 

 plan in all the following genera : — Apiocystis, Callocystis (PL I. 

 fig. 5), Cystohlastus (fig. 7), Glyptocystis (fig. 8), Lepadocrimis 

 (fig. 6), Pleurocystis, Primocystis, Pseudocrinus, and probably 

 also in Sphcerocystis and StroMlocystis ; and although in some 

 cases, e. g. Ecliinoencrinus, the radial character of these second 

 parabasals is not apparent at first sight, yet in types like Pseudo- 

 crinus and Apiocystis they are traversed by some of the ambu- 

 lacra, while in Cystollastus (PL I. fig. 7) they are deeply 

 incised by the latter, and are transformed into regular fork- 

 pieces, like the radials of the Blastoids, as already noticed by 

 Volborth §. The resemblance of all these types to one another 

 is such that if plates 10-14 of Cystohlastus be admitted as radials 

 (and this, I think, will scarcely be denied) the same name must be 

 extended to the centrolaterals or second parabasals of all the 



^ ' Paleontology of New York,' vol. ii. p. 223. 



t " On the OystideEe of the Silurian Eocks of the British Islands," Mem. Geol. 

 Survey Great Brit. 1848, vol. ii. part 2, p. 487. 



\ " Ueber die Arme der bisher zu den armlosen Crinoiden gezahlten Bchino- 

 encrinen," Bull. Class. Phys.-Math. Acad. Imp. Sci. St. Petersbourg, 1844, 

 tome iii. No. 6, p. 2 (of separate copy). 



§ "Ueber Achradocystites und Cystohlastus, zwei neue Crinoideen-Gat- 

 tungen," Mem. Acad. Imp. Sci. St. P6tersbourg, 1870, tome xvi. no. 2, p. 12. 



