THE CRINOIDEA 



159 



stem circular in section. Dichocrinus, Miinster (1838, W. & Sp., 1897, 

 syn. Cotyledonocrinus, Casseday & Lyon, 1860), Carboniferous, Europe and 

 N. America ; BB 2 ; IBr 2, united by syzygy ; IIB^ ^ 2 and, when arms 

 fork again, IIIBrj and 2 are united by syzygy ; arms thin, uniserial or 

 biserial, occasionally pendent ; stem circular in section. Camptocrinus, 

 W. & Sp. (1897), Carboniferous, America, differs from Dichocrinus 



Fia. LXXII 

 Hexacrinus. 1, analysis of cup ; 2, //. pateraeformis, cup from below. (After L. Schultze.) x L 



only in the structure of the stem, which in its crescentic section and 2 

 series of cirri resembles that of Herpetocrinus (p. 147). Talarocrinus, W. 

 & Sp. (1881-97), Carboniferous, N. America ; differs from its ancestor 

 Dichocrinus in its more massive plates and in having but one IBr to an 

 arm, and that small ; anal resembles ant. R in shape and size ; arms 

 branch twice, biserial, free from IIBr inclusive. Pterotocrinus, Lyon & 

 Casseday (1859, W. & Sp., 1897 ; syn. Asterocrinus, Lyon non Miinster), 

 Carboniferous, N. America ; a remarkable modification of Talarocrinus, 

 with Br up to IIlBr incorporated in cup ; large 

 wing-like processes spread out from tegmen, and 

 probably represent the hypertrophied axillary 

 lAmb. With this exaggerated type, the nearest 

 approach to the true Camerata, the Hexacrinidae 

 become extinct. FAMILY 3. ACROCRINIDAE. Cup 

 formed of 2 BB, forming a hexagon ; 5 RR ; 1 

 anal in line with RR ; and a large belt of acces- 

 sory plates between BB and RR. Acrocrinus, 

 Yandell (1855, W. & Sp., 1897), later Carboni- 

 ferous, N. America (Fig. LXXIII.) ; derived from 

 Dichocrinus, which it otherwise resembles, by the 

 gradual intercalation of 6-20 circlets of supple- 

 mentary plates, " superbasals " (SB) ; the SB im- 

 mediately above BB are always the latest formed 

 and the smallest ; the SB supporting the anal and 

 ant. R are in single series, the rest alternate. 

 In A. amphora the arms were recumbent on the cup and apparently 

 immovable. This remarkable family was the last to appear, and sur- 

 vived all other Adunata and all Camerata. 



ORDER 3. Monocyclica Camerata 



( = CAMERATA, W. & Sp. pars). 



Monocyclica in which IBr, two in each ray(exc. Stereocrinus, Hadrocrinus^ 

 Alloprosallocrinus) and often succeeding orders of Br, are incorporated by 



Fir.. LXXlIf. 



Aerocrinns amphora. (Re- 

 constructed from Wachs- 

 niuth & Springer.) X J. 



