viii ECHINODERMATA SYSTEMATIC REVIEW 309 



costals of the 5 rays laterally distinct. Interradials in two circles ; in the first 

 circle there are two plates to each interradius, and in the second circle only one, 

 which, however, is larger than the former two. Posterior interradius much larger, 

 with twice as many interradials, between which there is, further, an intercalated 

 vertical row of anal plates. Acrocrinus. 







Family 10. Barrandeocrinidae. 



Base monocyclic. 3 basals. The first anal plate rests on the circle of basals. 

 The interradials rest upon the sloping oral ends of the radials. Arms bent back on 

 the calyx, fusing laterally with one another by means of their pinnulse in such a 

 way as to form a firm envelope around the calyx. Barrandeocrinus. 



Family 11. Eucalyptocrinidse. 



Base monocyclic. The apical capsule consists of 4 basals, 5 radials, 2x5 

 costals, 2 x 10 distichals, 3x5 interradials, and 1x5 interbrachials. Xo anal 

 plates. The tegmen calycis consists of 5 large interradials, 5 large and 10 small 

 interbrachials, the oral plates, and two other plates lying further up towards the 

 apex. Anus shifted quite to the centre. The plates of the tegmen form 10 niches ; 

 in the bases of these niches ambulacral grooves (two in each) run to the bases of 

 the 10 pairs of arm-branches, which are received into the niches. Eucalyptocrinus. 

 CktUierinus. 



Order 3. Articulata (Ichthyocrinidae). 



Skeleton flexible. Anal plates often occur in the posterior interradius of the 

 calyx. Base dicyclic. Three infrabasals of unequal size, which are usually hidden 

 by the uppermost joint of the stem. Radials perforated, with one or more 

 primai'y brachials. The circle of the combined radials and primary brachials is 

 closed, or else interrupted by one or more plates in each interradius. The brachials 

 of the first, second, and often also of the third order are incorporated in the calyx. 

 The radials and the separate brachials are articulated together. Arms uniserial. 

 Pinnule appear to be wanting. Interradials irregular and varying in shape, size, 

 and arrangement, inconstant (may be either present or wanting in one and the 

 same species). In the posterior interradius there is often one asymmetrical plate. 

 Tegmen calycis only known in a few forms, soft and flexible, the plates lying in it 

 not being firmly fused together. Five separate orals of unequal size grouped round 

 the open mouth, the posterior oral being the largest. Ambulacra with their cover- 

 ing plates appear at the surface. Between them, there are interambulacral plates 

 which are occasionally distinguished by their remarkable size. Interambulacral 

 areas often sunk. Food grooves of the arms enclosed by movable covering plates. 

 A plated process (anal tube with anus ?) is found excentrically in the posterior 

 interradius of the tegmen. 



Fam. Ichthyocrinidae Palseozoic forms : Ichthyocrimis, Forbesiocrinus, Gleio- 

 cri iiu.s, Taxocrinus (Fig. 255), etc. 



The unstalked genus Uintacriiius, from the upper Chalk, and the extant unstalked 

 genus Thaumatoci-inus (Fig. 256), ought probably to be classed here. In the latter 

 the uppermost ossicle of the stem is retained as centrodorsal. The dorsal cup 

 consists, apart from the centrodorsal, of 5 basals, 5 radials, and 5 interradials, 

 which last rest on the circle of basals, and alternate with the radials. Tegmen 

 with central open mouth, which is protected by a pyramid of 5 large separate 

 orals. Between the orals and the edge of the calyx (or the oral edge of the 

 interradials of the dorsal cup) the tegmen is covered with small irregular plates 



