128 



THE CRINOIDEA 



FIG. XLII. 



The most conflicting views have been held from time to time 

 by the same and by different writers as to the homologies of these 

 plates. That here put forward agrees in the main with Neumayr's 

 (1889), but is based on facts not accessible to him. Wachsmuth 

 & Springer (1897) deny the homology of the 

 deltoids in Eublastoidea, Hybocrinus, and Cya- 

 thocrinidae, with the orals in Haplocrinus and 

 Antedon ; the plates here regarded as enlarged 

 ambulacrals (e.g. in Eublastoidea, Cyatho- 

 crinidae, Fig. XLIIL, Crotalocrinus) are taken 

 by them to be orals, and they imagine that 

 they undergo resorption, fission, and other 

 changes, stating that they are relatively larger 

 Tegmen of Megistomnus in young specimens. As to the origin and 



nodosiis, showing radial , i .1 i , TIT, 



dome -plates, d. (After homologies of the large mterradial plates in 

 Xw!T xt. & Springor ' Inadunata (here called A or 0), those authors 



are undecided. 



The gradual sinking of the ambulacra and their covering-plates 

 below not only the orals but other tegminal plates, has given rise 

 in the typical Camerata to structures so differentiated that they 

 were long misunderstood, and their chief elucidator, Wachsmuth, 

 believed in 1877 that the tegmen of Palaeozoic crinoids was "a 

 solid vault or dome," which could not " in the remotest degree be 

 homologised with the soft 

 peristome of" recent crinoids. 

 "It forms," he said, "a part 

 of the abactinal system " ; "a 

 continuation of the radial and 

 interradial series of the dorsal 

 side, and serves merely as a 

 covering and protection for 

 the organs underneath." 

 From this it was generally 

 inferred that an originally 

 flexible tegmen (" disc " it was 

 called, as in recent crinoids) 

 had been overgrown by "a 

 free arch which braces the 

 entire oral side of the body 

 without the aid of oral plates" (W. & Sp. 1881). The disc 

 remained as an " inner test," in which were ambulacra and 

 possibly orals. Because of this structure, supposed to obtain to a 

 greater or less extent in all Palaeozoic crinoids, but not in their 

 successors, the Crinoidea were divided into Palaeocrinoidea and 

 Stomatocrinoidea, the latter term being altered by P. H. Carpenter 

 to Neocrinoidea. 



FIG. XL1II. 



Tegmen of Cyathocrinus ramosus. The large 

 tegminal plates are not homologous with the del- 

 toids, but the squarish central one may be the 

 madreporite. (From Bather, 1893.) x 3. 



