118 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



in the tegmen, enables us to discriminate between perisomic and vault pieces; 

 and this led us to inquire whether such a distinction actually exists in any 

 group. 



After it was demonstrated that the ventral surface of Taxocrinus is a true 

 disk, we became convinced that our views respecting vault and disk would 

 have to be modified as to other groups also, or be altogether abandoned. The 

 latter was done in 1892, in a paper on " The Perisomic Plates," ^ in which 

 we endeavored to show that the so-called "vault" of the older Crinoids 

 is a modified disk. This interpretation has since been accepted by Agassiz, 

 Carpenter, and Bather, who agree that the use of the term ^' vault" should 

 be abandoned in a morphological sense. The modifications that took place 

 are most apparent among the Camerata, in which there is a marked increase 

 in the size and rigidity of the plates, which reached its culmination among 

 Carboniferous forms. 



The tegmen of Paleozoic Crinoids is often formed into ridges, which 

 diverge from near the centre to the arm bases. These ridges, which are 

 best defined, and occur most frequently, among Silurian Camerata, are formed 

 either by covering pieces or the interambulacral plates. Such ridges also 

 occur upon the disk of the Comatulae, but they are formed exclusively by 

 the ambiilacral plates. Among the earlier forms ridges of this kind have 

 been observed in Actinocrinus qumquangidaris^f Hahrocrinus ornatus.^X Marsu- 

 piocrinus depressus,\ Marsupiocrinus radiatus (Plate YIII. Fig. 15) ; and 

 Platycrinus symmetricus (Plate LXIX. Fig. 1^) ; in all of which the mouth 

 is closed either by the orals, or in their absence by the uppermost covering 

 pieces, which interlock with those of adjoining rays. Very prominent ridges 

 occur also upon the disk of Taxocrinus intermedius (Plate III. Fig. 11), in 

 which, contrary to the preceding forms, mouth and food grooves are opened 

 out. The ventral structure of this species bears a remarkable resemblance 

 to that of the young Platycrinus symmetricus (Plate LXIX. Fig. Ic) ; all 

 that is required to convert the "vault" of this Platycrinus into a disk like 

 Taxocrinus, is that its orals should be parted enough to let the ambulacra 

 pass in to the centre between or over their edges. Its resemblance is equally 

 striking to the recent Oalamocrinus and Hyocrinus ; a slight receding of the 

 posterior oral and movable covering pieces would bring the three forms sub- 

 stantially into the same condition, all of which shows that the closure of the 



* Proceed. Acad. Nat. Sci. Phila., pp. 345-375. 



t Iconogr. Crin. Suec, Plate XVI.,Pig. 28. % Ibid., Plate XXVH. Fig. 5. 



§ Ibid., Plate X. Fig. 16. 



