40 SMITHSONIAN INSTITUTION 



Mr. Clark, in the paper cited, said : " It is now known that all Recent 

 crinoids, with the single exception of Hyocrinus, are dicyclic " ; and in my 

 paper On a New American Jurassic Crinoid above cited, I stated as the 

 accepted conclusion from the researches of authors to that date, 1909, that 

 " there can be no longer any doubt that the Pentacrinidae are all either actually 

 or potentially dicyclic, though in some species the infrabasals are resorbed at 

 an early stage." The last observation had reference to Clark's further re- 

 searches in 1908, 1 in which he examined 15 specimens of Isocrinus decorus and 

 three of the closely related Endoxocrinus parrae (olim Pentacrinus muller'i). 

 He found infrabasals in all of the former, but none in the latter, in which, 

 however, there is a large hole in the place which they should occupy. This con- 

 dition is comparable with that which I have found in the species of Ichthyo- 

 crinus with infrabasals resorbed. 



As to the Basals, the one noteworthy fact in the Flexibilia is the frequent 

 great enlargement of the posterior plate in those forms having a well-developed 

 anal side. This occurs in many genera and in all the families, — for example in 

 Calpiocrinus, Forbesiocrinus, Dactylocrinus, Euryocrinus, Synerocrinus, Am- 

 phicrinus, and all the Taxocrinidae. Among those in which the posterior basal 

 is not materially larger than its fellows are Lecanocrinus, Mespilocrinns, Anis- 

 ocrimis, Pycnosaccus, Asaph ocrinus and Hornwcrinus. In all of the latter the 

 anal x is a prominent plate, whereas in those forms with elongated posterior 

 basal the anal x has lost its identity as a feature in the calyx. In many of these 

 cases the bulk added by the elongation of the basal is substantially equivalent 

 to that of the vanished anal x, and it is a significant fact that if we compare a 

 form like Lecanocrinus or Pycnosaccus (Pis. I, fig. 34; XII, fig. 10a), having 

 a very large anal x rising to the height of the radials and a low oblique radianal, 

 with one like Forbesiocrinus or Taxocrinus (Pis. XXIII, fig. 2; LIII, fig. 2b), 

 it will be found that when the radianal is carried upward from its position on 

 the right shoulder of the basal by the enlargement of that plate to the extent 

 of replacing x, it will occupy the same position relatively to the basal, but at 

 the level of the top of the radial instead of the bottom. 



The elongation of the posterior basal is usually accompanied by a certain 

 asymmetry, so that when this plate is angular distally, as in Forbesiocrinus, 

 the right 'sloping face is the longest (PI. XXIII, figs, la, 2, 3), and when it 

 supports a tube-like anal series, as in Taxocrinus, the socket in which the series 

 begins is not medially located, but lies more or less toward the right side of the 

 plate (PI. LIII, figs, la, 2b). Thus the first plate of the anal tube series in 

 both cases, instead of following the basal in direct succession, has usually an 

 oblique position between the right shoulder of the basal and the upper corner 

 of the adjacent radial, representing a late stage in the migration of the radianal 



1 Proc. U. S. National Museum, vol. 35, p. 87. 



