THE CRINOIDEA 



153 



Missouricrinus, S. A. Miller (1891), Burlington group, if truly Mono- 

 cyclic, must be placed near here. 



FAMILY 12. PLICATOCRINIDAE. Monocyclica Inadunata, with BB fused 

 into a knob-like support ; 4 or 6 (exceptionally 3, 5, 7, or 8) simple RR, 

 enclosing wide thecal cavity, each supporting an arm, which forks on. 

 IBr : ; IIBr wedge-shaped, united by perforate articulation, not, so far as 

 known, by syzygy, regularly pinnulate ; pinnulars tend to fuse. Tegmen 

 unknown. Columnals cylindrical, with radiately striated joint-surface. 

 For full account, and for the evidence of the axial canals passing from RR 

 into the fused basal circlet, so proving that it is no columnal, see Jaekel 

 (1893). Genera Plicatocrinus, Miinster (1839), Upper Jura, Germany 

 (Fig. LXVL), thin RR enclose a wide and deep thecal cavity, facets from 

 i to 4 width of R, crescentic, without 

 large muscle -plates ; proximal pinnules 

 composed of 3 pinnulars, which in suc- 

 ceeding pinnules are fused to a solid piece. 

 Tetracrinus, Munster (1839), Upper Jura, 

 France and Germany, has thick RR with 

 strong muscle-fossae. The occasional oc- 

 currence of 3, 5, 7, or 8 rays shows that 

 the normal 4 and 6 arose from the more 

 usual 5 as sports (i.e. discontinuous meris- 

 tic variations). FAMILY 1 3. HYOCRINIDAE. 

 Monocyclica Inadunata, with 3 thin BB, 

 the smaller one in 1. post. IR ; 5 RR, thin, 

 broad, and spade-shaped, with a slight axial 

 fold running straight up the middle from 

 the subjacent B and ending in a narrow 

 facet ; 5 arms, bearing unbranched alter- 

 nating ramuli, which may be modified in 

 their proximal portions for reception of surface with one outstretched arm 



r . -r, i 11 complete. (Adapted from Zittel, 



ripe gonads; Br long, cylindrical, with iss2, x l'.) 

 deep, narrow ventral groove ; below the 



first ramule, which is on the right of each arm, are 6 Br, with the 

 joint between each pair a syzygy ; between successive ramuli are 3 

 Br, united by syzygies ; proximal ramuli the longest, and the follow- 

 ing ones proportionately shorter, so that they all terminate on the 

 same level as the arm ends ; 5 triangular 0, separated from RR by 

 iAmb and Amb, cover the wide mouth and circumoral tentacles ; each O 

 as a rule pierced by a water-pore, and post. by 2 ; water-pores also 

 pierce several iAmb, but not in post. IR ; anal tube reduced to a small 

 cone towards the left of the post, interambulacrum ; columnals cylindrical, 

 slightly higher than wide, united by discs of ligament- fibres, joint- 

 surfaces hollowed and plain or indistinctly striate, with stellate axial 

 canal ; no cirri ; root unknown. The sole living representative of the 

 order is the unique species Hyocrinus bethellianus, W.-Thomson (1876, 

 Fig. LXVIL), dredged by H.M.S. Challenger 30 miles W. of Crozet Is., 

 in the Southern Ocean, while columnals are said to have been found 

 in Mid-Atlantic just X. of the Equator; see P. H. Carpenter (1884). 



lAi- 



B 



Fin. LXVI. 

 Plicatocrinus Fraasi, from aboral 



