THE CRINOIDEA 145 



The Carboniferous Belemnocrinus simulates many Dicyclica in the com- 

 plexity of its anal tube, but in arm-structure may be regarded as inter- 

 mediate between the Heterocrinidae and certain Neozoic Monocyclica. 

 Of these latter, the first to appear are the Plicatocrinidae, and from them 

 spring the Jurassic Saccocoma and the Recent Hyocrinus. 



FAMILY 1. HYBOCRINIDAE. Monocyclica Inadunata, in which RE. differ 

 but slightly in size and shape, except r. post. R, which supports on its 

 left shoulder the proximal plate (x) of the rudimentary anal tube, when 

 present, while its right shoulder is separated by a slanting suture from 

 the lower part of the plate and bears an arm. BB, 5. Arms, 3-5, 

 unbranched, uniserial, non-pinnulate, less wide than RR. O, so far as 

 known, large, with traces of hydrospires, not separated from RR by 

 supplementary plates ; post. cut into by anus and pierced by hydropore. 

 Amb rest on adjacent edges of 0. Genera Hybocystis, Wetherby (1880, 

 see p. 95, Fig. I.); Hybocrinus, Billings (1857, -59; syn. Indianocrinus, 

 Mill. & Gurl.), Ordovician and Silurian, N. America (Figs. LVII. 3, VI. 

 and XXXVI.) ; Hoplocrinus, Grewingk (1867), Ordovician, Russia, includes 

 Baerocrinus (Fig. LVII. 1, 2, 4). For chief literature see W. & Sp. (91). 



FIG. LVII. 



Hybocrinidae. 1, HopJocrinus dipentas (type) ; 2, Hoplocrinus dipentas (var.); 3, Ilybot-riniis 

 tumulus; 4, " Ilaerocrimts" Ungerni. 



FAMILY 2. STEPHANOCRINIDAE. Monocyclica Inadunata, with 5 equal RR, 

 each bearing an arm in a deep radial sinus. BB, 3, the unfused B being 

 usually r. ant. as in Blastoidea. Arms branched. large, with traces of 

 hydrospires (?) at their junction with RR ; anus at junction of post. with 

 RR, with minute valvular covering. Amb, often* fused, rest on adjacent 

 edges of 0, and 5 large ones, often fused, cover the peristome. Genus 

 Steplianocrinus, Conrad (1842, see p. 96, Fig. II.), Silurian, N. America 

 and Britain, and as " Rhombifera mira" in Bohemia. The arms are 

 said to branch, but no two observers agree as to the mode of branching, 

 and in fact it seems to differ in the various ambulacra of the few 

 specimens that preserve the arms (R. P. Whitfield in litt.). 



FAMILY 3. HETEROCRINIDAE. Monocyclica Inadunata, with x usually 

 resting on left shoulder of r. post. Rs, and partly on right shoulder of 

 1. post. R, but sometimes sunk between the two, and always supporting a 

 long anal tube. BB, 5. Arms, 5, with proximal Br usually full width of 

 RR ; isotomous or heterotomous, or bifurcate with armlets ; non-pinnu- 

 late. 5 0, without perisomic plates, have been observed in Heterocrinus 

 Jieterodactylus juvenis, but the relations of Amb to tegmen are unknown. 

 Genera locrinus, Hall (1866), Ordovician, N. America (Figs. XXVI. 1; 

 XXVIII. ; and LVIII. 1) ; primitive in its isotomous arms, and RR similar 

 in shape and undivided, except r. post. R, from which an upper portion, 

 in shape like an axillare, is separated by a horizontal suture, and bears an 

 arm on its r. shoulder, and on its 1. shoulder a line of ossicles supporting 



