THE CR1NOIDEA 



ably these genera descended, probably differed in the same way ; 

 and resemblances, undoubtedly of secondary nature, should not 

 lead us to place together forms of diverse origin. The distinction to 

 be drawn between monocyclic and dicyclic genera is more obvious 

 in the simpler crinoids ; but here too. there are parallel stages 

 passed through the monocyclic locrinus and Heterocrinus (p. 145) 

 correspond with the dicyclic Merocrinus and Ottawacrinus (p. 178) 

 respectively. Since the presence or absence of infrabasals is cor- 

 related with the radial or interradial position of the lobes of the 

 chambered organ, the derivation of one type from the other 

 involves more change than the mere atrophy or appearance of 

 certain plates. Hence monocyclic and dicyclic genera should not 

 be placed in the same line of descent, unless this change can be 

 proved : there is no reason why they should not have been inde- 

 pendently evolved. The origin of Dicyclica from Monocyclica 

 is, in fact, opposed by the available evidence ; but Monocyclica 

 may conceivably have been derived from Dicyclica in one of two 

 ways, outlined in the next two paragraphs. 



There are, especially among the later crinoids, several genera 

 known as " pseudomonocyclic," because though infrabasals are in- 

 visible or absent, at all events in the adult, the evidence of the 

 axial cords (e.g. in Rhizocrinus) of palaeontology (e.g. Apiocrinus 

 and Pentacrinus), or of embryology (e.g. Antedon), demonstrates the 

 existence of infrabasals either in the young or in near ancestors. 

 Discoveries of this nature have strengthened Wachsmuth & 

 Springer's law by affording a rational explanation of apparent 

 exceptions. But suppose secondary growth of stereom to occur 

 in a pseudomonocyclic genus, converting the angles of the stem 

 from interradial to radial, and the angles of the stem-lumen from 

 radial to interradial. Then, if recent examples were known, the 

 law, as emended above (p. 106), might be applied successfully ; 

 but it would not tell the truth if only fossils were available, and 

 the crinoid would pose as monocyclic. Such changes are actually 

 observed in the growth of Antedon, while in Isocrinus, which other 

 facts prove pseudomonocyclic (Fig. XL), the angles of the stem- 

 lumen in the proximal region are interradial, as if the genus were 

 truly monocyclic, though the downward prolongations of the 

 chambered organ are radial. In Glyptocrinus Fornshelli the angles 

 of both stem and axial canal are radial ; since there are actually 

 no infrabasals, we may suppose secondary ingrowth of stereom, 

 and this is confirmed by S. A. Miller's description of the columnals 

 (1874). 



The changes just described leave the essential distinction 

 between monocyclic and dicyclic genera untouched (as shown in 

 Fig. XII.) ; and among the earliest crinoids there is little evidence 

 of pseudomonocyclic forms. There is, however, a possibility that 



