THE CRINOIDEA 



147 



traces of original pentarnerism, but over the greater part of the stem are 

 crescentic in section, the concavity being on the inner margin. Cirri, 

 borne on the horns of the crescent, vary in their arrangement in different 

 species. No root in adult. One ray missing ; other RR all compound, 

 except 1. post. R sometimes ; x lower than in other Heterocrinidae, partly 

 rests on r. post. Ri (the radianal, RA). The tube outwardly resembles a 

 series of Br and covering -plates. Arms slightly heterotomous. The 



FIG. LIX. 



Herpetocrinus Fletcheri, in its natural coiled position, the cirri which covered the crown 

 having been removed. C, cirri ; S, longitudinal suture of stein ; t, anal tube. (After Bather, 

 1893.) x 5. 



remarkable resemblance of the coiled cirriferous stem to a pinnulate, 

 canaliculate arm has misled most writers ; for the crown is rarely visible 

 (vide p. 134). FAMILY 4. CALCEOCRINIDAE. Monocyclica Inadunata, with 

 the essential characters of typical Heterocrinidae, the 1. ant., r. ant., and 

 r. post. RR being compound, and the arms branching primitively on the 

 plan of Heterocrinus heterodactylus ; but with the following modifications 

 induced by the bending down of the crown : the r. post. IR lies alongside 

 the stem ; the 1. ant. R lies away from the stem ; the plane thus marked 

 is one of a gradually increasing bilateral symmetry ; r. post, arm always 



