174 



ECHINODEKMATA— PELMATOZOA 



PHYLUM IV 



plates at the base of the ambulacra or arms. When the base is monocyclic 

 (Fig. 268) the position of the proximal ring of plates is interradial ; but when 



Fig. 267. 



Euspirocrinus spir- 

 alis Ang, Stalked 

 Crinoid with dicyclic 

 base and anal inter- 

 radius. a, Anals ; b, 

 Basais; ib, Infrabasals; 

 r, Radiais. (Right and 

 left sides reversed, 

 after Angelin.) 



Fig. 268. 



Cactocrinus 'probosckiialis (Hall). Prqjec- 

 tion of calyx sliowing the tliree basals (5), 

 5x3 simple radials (r), four iiaired interrays 

 (ir), and a tifth unpaired anal interray (a). 



Fig. 26'.). 



Pachylocrinus multiplex 

 (Traut). Calyx with dicy- 

 clic base, radials, costals, 

 and distichals. 



dicyclic it is radial, and the upper ring corresponds with the basals of mono- 

 cyclic forms (Fig. 269). 



In the nomenclature of P. Herbert Carpenter, the upper series of plates in 

 the dicyclic base are properly termed basals, and the lower series infrabasals 

 iunderbasals). The basals as thus defined are equivalent to the "parabasals" 



in the older nomenclature of Johannes Müller, 

 and to the " subradials " of de Koninck and 

 other authors. 



Both basals and infrabasals are primarily five 

 in number ; but owing to the supposed morpho- 

 logical (rarely if ever actual physical) fusion of 

 two or more of the plates, the number of basals 

 in the monocyclic forms may be reduced to four, 

 three, two, or even to a single undivided plate ; 

 and that of infrabasals in the dicyclic to three. 

 During the ontogenetic developmentof the Recent 

 Antedon, a more or less complete resorption of the 

 basals has been observed, which ultimately results 

 in their passage from the dorsal to the ventral 

 side of the so-called chambered organ, where they 

 are again rebuilt, becoming a curious plate-like structure known as the 

 rosette; and the same probably also was true for certain Mesozoic genera 

 (Eugeniacrinus, Phyllocrinus). In many of the non-pedunculate Crinoids 

 {Uintacrinus, Marsupites, Fig. 270) an additional plate known as the centrale 

 rests against the infrabasals, and probably represents an undeveloped stalk. 

 The bg,sals are united with one another and with the overlying radials by 



Fig. 270. 



Marsupites testudinaris (Schloth.). 

 Diagram of calyx. cd, Centrale ; ib, 

 Infrabasals ; b, Basals ; r, Radials. 



