126 



THE CRINOIDEA 



The generally accepted view that these are orals is confirmed by 

 the frequent presence of a pore or pores in the posterior one, as in 

 larval Antedon and adult Hyocrinus, and by the situation of the 

 anus between this plate and the adjacent RR ; in Haplocrinus the 

 pore and the anus appear to be combined near the oral end of this 

 plate. These plates have in preceding pages been spoken of as del- 

 toids (A). They meet close around the mouth like the A of many 

 Blastoids, and in Hybocrinus and Carabocrinus they show traces of 

 hydrospires of Cadaster type. On the other hand, they are 

 homologised, and justly so, with five similar plates that occur in 

 Coccocrinus, Symbathocrinus, Pisocrinus, Allagecrinus, Myrtillocrinus, 



some Platycrini, and the specimen of 

 Taxocrinus intermedius (Fig. XXXVII.) 

 described by Wachsmuth & Springer 

 (Nov. 1888). In most of these 

 genera the orals (or A) cover the 

 mouth, and the food -grooves pass in 

 under them ; but in Taxocrinus the 

 mouth is open, and the grooves with 

 ambulacrals pass between the orals ; 

 while in Hybocrinus and Carabocrinus 





FIG. xxxvn. 



Tegmen of Taxommis intermedius. 

 A, anal ridge; Jir, edges of brachials; 

 iAmb, UIAmb, HIIAmb, interainbula- 



o^ !STSrtt ll ftS 

 * 8 * 18 (Alter Wachsmuth & 



the apposed orals, showing that the 



grOOV6S Were actually above those 

 . 



plates. 



From this primitive Palaeozoic type, 

 three lines of evolution start: (1) 

 Ambulacra pass over the edges of the 

 orals, while ambulacrals and sometimes interambulacrals gradually 

 cover the orals, which seem thus to sink below the surface and to 

 diminish in size ; the posterior oral, however, usually remains large 

 and is pierced by hydropores, while the increased size of the anal tube 

 pushes it more towards the oral centre (e.g. Euspirocrinus, Cyatho- 

 crinus, Cupressocrinus, Figs. XXXVIII., XXXIX.). (2) Ambulacra 

 and ambulacrals pass between the orals, leaving an open mouth, 

 while the orals gradually atrophy (e.g. Taxocrinus, Fig. XXXVII., 

 probably other Palaeozoic Flexibilia, and certainly many of their 

 Neozoic descendants). (3) Ambulacra pass beneath the orals, 

 and gradually also beneath other tegminal plates, which are 

 developed pari passu with the incorporation of brachials in the 

 cup, and which thus separate the orals from the periphery 

 of the tegmen (e.g. Adunata and Camerata, Fig. XL. ; cf. Caryo- 

 crinidae, p. 66). 



At the same time, in types (1) and (3) a modification of the 

 ambulacrals takes place. The proximal ambulacrals covering the 

 mouth in (1) become large, and assume a pentagonal arrangement 



