358 PROCEEDINGS OF THE ACADEMY OF [1888. 



The iiiterradials in the Apiocrinidae, extending up between the 

 rays, connecting with, and forming a part of the ventral covering, 

 find a close parallel in those of many of the Ichthyocrinidae, and 

 since the discovery of a disk and open mouth in Taxocriiais, we 

 have not the slightest doubt, that these plates represent the same 

 elements in both groups, forming in both of them parts of the disk, 

 and that perhaps the same is the case with the interradials and 

 interaxillaries of Uintacrinus, which in many respects resemble those 

 of the Ichthyocrinidae. 



The subtegminal mouth, which we supposed to be the best char- 

 acter of the Palaeocrinidea, proves to be subject to exceptions fully 

 as great as the others. Our recent discoveries show that in some 

 palaeozoic crinoids, and probably in the Ichthyocrinidae generally, 

 the mouth is exposed, and there is no vault aside of the orals; and 

 we are not certain but that we may find other exceptions among 

 the later Poteriocrinidae and Encrinidae. We now know that there 

 are no additional elements in the oral system of palaeozoic crinoids, 

 but that the mouth opens out in a very similar manner by the part- 

 ing of the orals as in the larva of recent forms, and this leads us to 

 put less faith than before in the condition of the mouth as a char- 

 acter for the subdivision of the Crinoidea. For these may well be 

 different stages in the development of the mouth, represented in 

 palaeontological time, and we need not be surprised to find at some 

 time a Silurian Ichthyocrinoid with the orals closed, or a Haplo- 

 crinoid with the orals parted. 



From this review of the principal characters relied upon to dis- 

 tinguish the earlier from the later crinoids, it will be apparent that 

 the exceptions are so numei'ous as to leave nothing stable or definite 

 on which to base such important primary divisions, and we are again 

 confronted with the problem of rectifying the classification of the 

 Crinoidea, or proposing a new one. It is true that many of these 

 exceptions are due to differences which tend to se|)arate the Ichthyo- 

 crinidae from the Palaeocrinoids, and unite them with the Neo- 

 crinoids ; and it might be the simplest, as well as the least radical 

 change, to modify the definition of the Neocrinoidea so as to admit 

 the Ichthyocrinidae, which would thus fall exactly into that place 

 among them, for which Carpenter was always obliged to make an ex- 

 ception in favor of Thaumatocrinus. In so doing, however, we 

 would be bringing together some of the earliest and latest forms* 

 which would render the name Neocrinoidea wholly inappropriate. 



