1885.] NATURAL SCIENCES OF PHILADELPHIA. 253 



suggest, either that the condition of the ventral surface is of 

 comparatively little value for classificatory purposes, or that 

 certain forms, which have heretofore been described under Actino- 

 crinus, are structurally very different, and should be referred to 

 remote groups. It would further prove, if the upper interradial 

 plates in Platycrinus were anambulacral pieces — because some of 

 the covering pieces are interposed between them — that the higher 

 interradials of Actinocrinus stellatus, which are in the same con- 

 dition, are perisomic, and vice versa those of certain Platycrinida' 

 vault plates ; indeed, that the very same plates which in the youn» 

 Platycrinoid represent vault pieces, are perisomic in the adult. 



Carpenter will admit that the minute temporaiy interradials, 

 which Sir Wyville Thomson observed in the larva of Antedon, 

 are the homologues of the large and permanent calyx interradials 

 in the Cyathocrinidae. In this group, in which the rays are free 

 from the first radial, the interradials, for want of any other lateral 

 support, join with each other, and thereby attain their ventral 

 position ; while in the adult Actinocrinidae and Rhodocrinida\ 

 which have numerous radial and interradial plates, the first inter- 

 radials naturally had to be located dorsally. The increase of 

 interradial plates took place gradually in the growing animal and 

 from that we may reasonably suggest that these Crinoids at one 

 time in their larval state possessed but five single interradials 

 which met over the disk ventrally,as in the case of Cyathocrinus 

 alutaceus. At that time the 3 T oung Actinocrinus was essentially 

 in the condition Of a young Antedon in which the interradials had 

 made their appearance, however the interradials of the Palseo- 

 crinoid were more fully developed. If now Allagecrinus and 

 Haplocririus, as suggested by Carpenter, represent palaiontologi- 

 callv a very early stage of the larva of Antedon, we should like 

 to know something about the condition of the interradial plates 

 in those genera. Are they as yet contrary to all other Palaeo- 

 crinoidea altogether unrepresented, or here already resorbed by 

 the animal ? Both genera have five plates, which occupy the very 

 same position as the interradials of Cyathocrinus alutaceus, and 

 Cyathocrinus Gilesi (PL -4, fig. 2). Why should these be orals. 

 when there is another structure covering the tentacular vestibule 

 which may represent them, and which, on the other hand, would 

 be totally unrepresented in the Antedon larva and in all other 

 Echinoderms ? 



