58 THE CEINOIDEA CAMERATA OE NOETH AMEEICA. 



Hijocrinus and Rhizocrinus^ and it is possible that the former has really but three 

 basals ; but in Rhizocrimis the fusion was not complete^ as Count Pourtales 

 proved by inserting a needle in the central canal, whereupon the base sepa- 

 rated into five pieces."^ 



The foregoing facts are of considerable importance as to classification. 

 For if the smaller number of plates is merely a modification of the original 

 five, the number of basals cannot be regarded as a good character for sepa- 

 rating the Crinoids into primary groups, as was practically done by Angelin, 

 and afterwards by S. A. Miller. 



The basals of dicyclic Crinoids, as we have stated, invariably consist of 

 fiyQ pieces, but their form differs in different groups. In the Rhodocrinidse 

 (Fig. 17) they are all heptagonal; while in the Thysanocrinidge the posterior 

 one is heptagonal, and the four others hexagonal (Fig. 18) — the former 

 supporting a supplementary plate at each interradius, the latter at the anal 

 side only. In the Fistulata, all fiMQ plates are hexagonal when there is no 

 anal plate, as in the Encrinidge ; but in the Cyathocrinidae, whenever that 

 plate is represented, four of the basals are hexagonal, and the fifth hepta- 

 gonal, precisely as in the Thysanocrinidge. In the Poteriocrinidge, and all 

 other families in which in addition to the regular anal plate a radianal is 

 represented, only three of the basals are hexagonal, and the posterior and 

 right postero-lateral basals are heptagonal (Figs. 15 and 16). 



The same variations occur among the basals of the Ichthyocrinidge. 

 Those of IcUhyocrinus agree with the basals of the Encrinidge ; those of 

 Mespilocrimis, Taxocrinus, Forhesiocrimis, and Caljoiocrmus, with the plates of the 

 Cyathocrinidge ; while the basals of Lecanocrinus, Gnorimocrimis and Bagenocrimis 

 are in a similar condition to those of the Poteriocrinidge. 



Three infrabasals have been observed only among the Fistulata and 

 Articulata. They are represented sparingly among the former, but are the 

 rule among the Ichthyocrinidse, and are represented in the larva of the 

 Comatulge. Wherever they occur, they consist of two large plates and a 

 smaller one, which are differently arranged in the various groups. In the 

 Fistulata, according to Bather, the position of the small plate is not constant ; 

 we have always found it located anteriorly below the suture between the 

 two anterior basals, and the plates / and g and i and h fused together 

 (Fig. 16). In the Ichthyocrinidge, however, the small plate rests below the 

 suture between the posterior and right postero-lateral basal, and/ and ^ and 



* Memoirs Mus. Comp. ZooL, Vol. lY., p. 29. 



