62 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



stem ]omt o^ Millericrinus Orlignyi ^Vi& M, ^olydadylus ; while in other species 

 of the Apiocrinid^ in which infrabasals are unrepresented, the cohmm 

 touches the basals only. It rests against the outer (dorsal) surface of the 

 plates; whose loiver margins are bent up, so as to form an inverted pentan- 

 gular concavity, radially disposed. This cavity, which is occupied and 

 completely filled by the top stem joint, is generally grooved at the inter- 

 basal sutures, and produces upon the upper face of the joint five angles, 

 which fit into the grooves, and interlock with the basals (Plate YI. Figs. 1 a^ 

 1 ^, and 5). The outer edge of the joint at the upper end has also a more or 

 less radial outline ; while its lower end follows the orientation of the stem, 

 and is interradially disposed when that is pentangular. 



We mention these particulars, because Carpenter, in criticising our gen- 

 eralization,* undertook to prove by the angularities at the upper face of the 

 stem that in those species of the Apiocrinidae in which the stem is round, 

 the latter was radially disposed, and not interradially ; and that those 

 species, according to our own rules, were monocyclic and not dicychc. He 

 overlooked the fact that the top joint rests against the turned up dorml 

 (outer) surface of the basals, and not against their inferior faces. The surface 

 to which the " centro-dorsal" is attached, represents morphologically the 

 surface of the concavity for the reception of the column in Palaeozoic Cri- 

 noids ; the inferior faces are those which meet the infrabasals, and in M. Or- 

 hignyi and M. loolydadylus actually support them, but in most of the Apio- 

 crinid^ they enclose a vacant space. If this space was filled by infrabasals, 

 as we think it was in the young Crinoid, the columnar concavity and the 

 upper face of the stem would be in exactly the same condition as in the 

 Ichthyocrinidas, in which similar angularities occur on the upper face of the 

 stem (Plate II. Fig. 4^). The upper face of the column in all Crinoids 

 adapts its form to the shape of the plates to which it is attached, and if their 

 suture lines are grooved, it will be correspondingly ridged. We thus beheve 

 that Carpenter misunderstood the structure, and that he overlooked our 

 statement that it is the longitudinal angles along the column which alternate 

 with the proximal ring of plates in the calyx, and not the angularities or 

 ridges of the upper face. 



In Rhizocrinus the condition of the base is apparently similar to that of 

 the Apiocrinidae. The genus, however, was described by Carpenter! as 



* Ann. and Mag. Nat. Hist., March, 1886, p. 286. 

 t Chall. Rep. Stalk. Crin., p. 246. 



