CLASS III 



CRINOIDEA 



201 



§ 2. Basais two. 



Dichocrinus Münster (Cotyledonocrinus Lyon and Cass.). Costals two, 

 usually united by syzygy, as are also the first two brachials of each order, 

 giving the appearance of a single plate. Arms biserial, branching once to 

 three times ; occasionally pendent. Stern round. Lower Carboniferous ; 

 Belgium, Great Britain, North America (Kinderhook to Kaskaskia). 



Camptocrinus W. and Sp. Like Dichocrinus except in the stem, which is 

 concavo-convex in section, as in Herpetocrinus, tending to coil around the crown. 

 Lower Carboniferous (Burlington to St. Louis) ; North America. 



Talarocrinus W. and Sp. Costal one, small, trigonal, sometimes hidden 

 by the distichals. Calyx plates massive ; anal resembling anterior radial in 

 form and size. Column round. Lower Carboniferous (St. Louis to Kaskaskia) ; 

 North America. 



Pterotocrinus Lyon and Cass. (Asterocrinus Lyon non Münster). A re- 

 markable modification of Talarocrinus. Brachials up to third order incorporated 

 into dorsal cup. Anal plate much smaller than the radials. The axillary 

 first ambulacral produced into large wing-like processes, stretched out from 

 the tegmen. Lower Car- 

 boniferous (Kaskaskia) ; 

 North America. 



Family 11. Acrocrinidae 



Waclismutli and Si^ringer. 



Monocyclic. Basais separ- 

 ated from radials by a large 

 bell of accessory pieces. Badials 

 in contact except at the posterior 

 side, where they are separated 

 by an anal plate, Structure 

 otherwise as in the Hexa- 

 crinidae. Lower Carboni- 

 ferous. 



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 O 



O 9ä<o" 



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Acrocrinus Yandell (Fig. 

 303). Basais two, forming 

 a hexagon. Calyx as in 

 Dichocrinus with intercala- 

 tion of several circlets (u^) 

 to twenty) of supplementary 

 plates, those immediately 

 above the basals being the 

 smallest and latest formed. 



Costals small, trigonal. Arms two to four to the ray, biserial, either erect 

 or pendent. Column round. Lower Carboniferous. The last survivor of 

 the Camerata (Burlington to Coal Measures), and known chiefly in North 

 America but reported recently from England. 



/ R N 



Fio. 303. 

 Acrocrinus sp. Lower Carboniferous. /), Basals; R, lladials 



I, Costals ; x, Special anal plate. 

 ary plates (after W. and Sp.). 



tVU the others are Supplement- 



