CLASS III 



CRINOIDEA 



217 



forming a flexible network, which may be continuous all around the crown 

 or be divided into five broad, reticulate, fan-like fronds. In the calyx of 

 some young specimens the radial facet is narrow, semicircular, evidently 

 bearing the brachials in the usual succession. Axial canal distinct in arms 

 but not perforating radials, which are thin. Stem terminating in a thick, 

 branching root. Silurian ; Gotland, England and North America. 



Pefalocritius Weller, from the Silurian of North America and Gotland, 

 with its arms united by lateral fusion into five ponderous fans, has some 

 resemblance to Crotalocrinus, but seems to have no- anal plate. It may be 

 nearer to the Gasterocrinidae. 



Family 9. Cyathocrinidae Roeiiier (emend. Wachsin. and Springer).' 



DicycUc. Tegmen strong, composed of rather large orals more or less exposed, 

 surrounding but not covering the peristome ; rigid amlndacrals supported on their 

 adjacent edges, meeting above the oral centre and often greatly modified ; and inter- 

 amhulacrals, which often encroach upon and obscure the other plates. Posterior oral 

 frequently a madreporite. Amis located either in the posterior interambulacrum 

 directly through the tegmen, or at the distal end of a plated anal tube, or dor sally 

 through the side of the cup. Arms non-pinnulate. Badial facets usually semi- 

 circular, less than the width of the radial. Union of radials tvith proximal brachials 

 usually by incomplete articulation upon undifferentiated joint faces, with concavo- 

 convex surfaces, without true transverse ridge, though with occasional traces of it. 

 Infrabasals usually five. Stem usually round. 



Subfamily A. Carabocrininae. 



Arms usually dichotomous ; heterotomous in some of the later genera. Radianal 



obliquely to left of right posterior radial. Anal x present. Posterior oral usually 



a madreporite. Infrabasals five. Stem usually round. Ordovician to Lower 

 Carboniferous. 



m 



Carabocrlnus Billings. PiA completely separating BB, and having a 

 supplemental plate intercalated below it, touching IBB. Anal x large, 

 line with radials. Anus directly through the teg- 

 men. Arms branching. Posterior oral pierced by 

 hydropore. Ordovician (Trenton) ; Canada and 

 Kentucky. 



Strophocrinus Sardeson. Ordovician ; Minnesota. 



Porocrinus Billings (Fig. 314), BA smaller, rhom- 

 boidal, not separating BB. Arms ten, unbranched. 

 Calyx plates deeply folded at the angles, but folds 

 do not cross the sutures or form true pore-rhombs. 

 Anus in a slight protuberance. Eeferred by some 

 authors to the Cystids. Ordovician ; Canada, Ken- 

 tucky and Eussia. 



Palaeocrinus Billings. BA as in preceding genus. 

 Arms branching several times ; slender, rising from 

 a small curved facet. Anal tube small. Ordovician ; Canada and Kentucky. 



Homocrinus Hall. RA as in preceding. Arms branching, strong ; radial 

 facets wider than usual in the family, nearly straight. Anal tube large, 



Fig. 314. 



a, Porocrinus conicus Billings. 

 Ordovician ; Ottawa, Canada. 

 Nat. size (after Billings) ; b, P. 

 radiafus Beyr. Ordovician ; St. 

 Petersburg. Calyx plates show- 

 ing folds at angles. Considerably 

 enlarged (after Beyricli). 



