i 5 8 



THE CRINOIDEA 



move only in one plane, has a spiral twist that enables it to bend in any 

 direction (Fig. XLIX. 5, 6). Eucladocrinus, Meek (1871, W. & Bp., 

 1897), Carboniferous, N. America ; appears late in the history of Platy- 

 crinus, from which it was evolved polygenetically by modification of the 

 arms ; two main rami to each arm (sometimes only one ramus) composed 

 of biserial Br, with large almost rigid Amb, form tubular extensions of the 

 thecal cavity, and give off on alternate sides short, biserial ramuli, which 

 in turn bear pinnules (Fig. LXXI. 4). This sketch of the evolution of the 

 Platycrinidae is confirmed by the ontogeny of Platycrinus. In young 

 stages (Fig. LXXI. 2) the basal cup is relatively shallower ; RR less high ; 

 columnals circular in section ; Br uniserial, later on zigzag, and longer ; 

 pinnules relatively stouter and wider apart ; O relatively larger, and 



DAx 



i iAmb 



Fio. LXXI. 



Platycrininae. 1, aboral view of cup and proximal bracliials of Platycrinus subspinosus (after 

 Wachsmuth & Springer). 2, crown of young P. Huntsvillae, from Brit. Mus. E0778. x f . 3, 

 calyx of P. tminvlus, anterior view (after W. & Sp.). 4, arms of Kucladocrinus millebrachiatus, 

 oral surface showing covering-plates (c.7>)and aboral surface (after W. & Sp.). Br', articular facet 

 for arms ; Rm, ramus ; r, rainule ; other letters as usual. All (exc. 2) two-thirds natural size. 



occupying greater part of tegmen. From this obviously Coccocrinine stage, 

 the change to the mature Platycrinus has been observed in many species. 

 GROUP B. FAMILY 2. HEXACRINIDAE. Cup formed of 1, 2, or 3 BB, 

 forming a hexagon ; 5 RR ; and 1 anal in line with RR. The family 

 is closely related to the Platycrinidae, but differs in the hexamerous 

 symmetry ; also, as a consequence of this, in the presence of an interbasal 

 suture in post. IR, whereas in Platycrinidae the nearest to that position 

 is in r. post, radius. Genera Hexacrinus, Austin (1843, Schultze, 1867, 

 W. & Sp., 1897), Devonian, Europe and N. America (Fig. LXXII.) ; 

 BB 3 ; IBr, 2 united by syzygy in America, only 1 in Europe ; arms with 

 2 rami, bearing ramuli on one or both sides at intervals ; Br uniserial, 

 and all except axillaries bear pinnules ; tegmen as in Platycrinus ; stem 

 circular in section, with small round lumen. Arthracantha, Williams 

 (1883, syn. Hystricrinus, Hiude, 1885), has theca of same structure, but 

 armed with movable spines borne on tubercles ; arms dichotomous, biserial ; 



