128 



THE CRINOIDEA 



FJG. XLII. 



Tegmen of 

 nodomui, sliowing 

 dome - plates, d. 

 th & 



radial 



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 

 in young specimens. As to the origin and 

 (After homologies of the large interradial plates in 

 Spnnger, i nac ] una t a (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 



Fia. XLII I. 



Tegmen of Cyathocrinus ramosns. The large 

 tegminal plates are not homologous with the del- 

 toids, but the squarish central one may be the 

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



disc 

 and 



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

 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. 



