112 PROCEEDINGS OF THE ACADEMY OF [1887. 



need only refer to the parallel cases of Cyathocrinidae and Poterio- 

 crinidae. In the former, in which the articular facets were com- 

 paratively undeveloped, we have been able, in a number of instances, 

 to observe ventrally the interradial plates, which Etheridge and 

 Carpenter formerly regarded as structually identical with the so- 

 called orals of AUagecrinus. Whilein the Poteriocrinidae, in which 

 the articular facets are highly developed, no trace of these plates 

 has ever been found. 



We, of course, do not claim that this is positive proof, that in 

 Allagecrinns these plates were not carried inward by perisome, but 

 it militates strongly against the probability of such a thing, while 

 the theory that they were is at best but the merest conjecture. 

 If Etheridge and Carpenter had placed AUagecrinus and Haplo- 

 erinus among the Neocrinoidea as larval forms, they might be much 

 better warranted in supposing that the plates in question were orals, 

 and were afterwards carried inward, but both forms have been re- 

 ferred by them to the Palaeocrinoidea, in which that mode of devel- 

 opment is altogether unknown. The case of Ct/athocrmus shows 

 cleaidy that in the later types of the Inadunata the conditions of the 

 Palaeocrinoidea remain unchanged. The summit plates are not 

 carried inward by perisome, but occupy the same space as in the 

 earlier forms, and the perisome is formed upon the outer surface of 

 the interradials. (Revision, Pt. Ill, PI. IV, Figs, 2, 3, 6.) 



We should like to know upon what ground the authors maintain 

 that those genera are Palaeocrinoids, when they interpret their 

 structures according to the rules characteristic of the Neocrinoidea. 

 They neither have an anal plate, nor does AUagecrinus show any 

 such irregularity in the arrangement of its plates, as would of itself 

 warrant a reference to the Palaeocrinoids. The only irregularity 

 noticed in AUagecrinus is that the radials in some specimens may 

 be axillary in one to four of the rays, or not axillary in any o^ 

 them, and upon this character, curiously enough, Etheridge and 

 Carpenter seem to have separated AUagecrinus from the Haplocri- 

 nidae and made it the type of a distinct family. On this alone it 

 appears they divide it from the Neocrinoidea, as if it were one of the 

 most constant characters among the Palaeocrinoids ; while in fact 

 this peculiarity is found only in the Catillocrinidae, in two of the 

 rays of Tribrachiocrinus, and occasionally in AUagecrinus. A char- 

 acter like this is liable to be discovered exceptionally in any new 

 form of Neocrinoids, just as well as among Palaeocrinoids, while among 



