18T9.] NATURAL SCIENCES OP PHILADELPHIA. 243 



well with Carpenter's interpretation, the three basals being 

 divisible into five plates interradially arranged. 



All these facts seem to indicate that the "subradials," in genera 

 where the}' exist, are really the basals, and in such simple forms 

 as Cyathocrinus, it seems very reasonable to consider these plates 

 as the homolognes of the genitals, and the radials as the ocular 

 plates in the apical system of the Echini. In more complex forms, 

 such as the Actinocrinidse, Rhodocrinidx. Ichthyocrinidse, etc., 

 there would seem at first to be an objection to this interpretation, 

 arising from the fact that there are several other orders of plates, 

 both radial and interradial, within the body walls, and that in these 

 cases, as in Cyathocrinidee, we should find the homologues of the 

 apical plates of the Echini in the entire calyx, or the whole series 

 of plates of the aboral side up to the region of the arms, and not in 

 the two rings alone which Carpenter points out as such; in other 

 words, that the apical plates in the Echini cannot be homologized 

 with some few plates in the calyx of Paloeocrinoidea. 



In the younger stages of Paleocrinoids, the higher series of 

 radials are unconnected by interradial or axillary plates, as may 

 be seen most beautifully illustrated in the growth of Strotocrinus} 



It is also probable that at a still earlier period in the life of 

 these Crinoids, the second and third primary radials constituted 

 a free raj', as in the more simply constructed Gyathocrinus. In 

 Actinocrinus, etc., the basals, which according to Carpenter are 

 homologous to the "subradials" in other families, and the genitals 

 in Echini, develop very early in the young, and attain almost their 

 full size when even the first radials are comparatively much smaller. 

 We have in our possession a Cynthocrnnus, not more than half an 

 inch in length including the arms and a portion of the column, in 

 which, while the proximal plates are comparatively small, the so- 

 called " subradials" are developed to an extraordinary degree, far 

 more than the radials. The specimen in this stage looks remark- 

 ably like Billing's Lower Silurian genus Hybocrinus, in which the 

 first interradial ring of plates is enormously prominent and gib- 

 bous, while the proximal ring is apparently wanting, or if it exists, 

 is very minute. 



1 See our paper on " Transition forms in Crinoids," Proceed. Acad. Nat. 

 Sci. Philad., 1878, p. 233, and also pp. 229-235 for illustrations of develop- 

 ment in the parts in question in successive epochs. 



