THE CRINOIDEA 



165 



stem relatively large and -sub-pentagonal. Genus Tanaocrinus, W. & 

 Sp. (1897), Ordovician, Ohio (Fig. LXXVIII. 1, 2). In structure and in 

 time this genus is well fitted to be taken as an ancestor of the Carpo- 

 crinidae. FAMILY 2. XENOCRINIDAE. Batocrinoidea with 4 BB ; each 

 half-ray contains IIBr, and sometimes IIIBr ; free arms in zigzag or 

 biserial. iBr and illBr, also illlBr when present, numerous. Post. IR 

 wider than the others, divided by a longitudinal row of ridged anals. 

 Tegmen of minute irregular plates. Stem quadrangular to circular in 

 section, with pentagonal lumen. Genera Xenocrinm, S. A. Miller 



FIG. LXXVIII. 



Tanaocrinidae and Xenocrinidae. 1, Tanocriniis typm, posterior view, x 2 diam. 2, anterior 

 view, x 2 diam. 3, Xenocrinus, analysis of cup. 4, Compsocrinus harrisi, cup from posterior, 

 diagrammatic, x 2 diam. (All adapted from W. & Sp.) 



(1881 ; W. & Sp., 1897 ; Fig. LXXVIII. 3) ; iBr sink between RR, so 

 as almost to rest on BB ; IIBr are finials. Compsocrinus, S. A. Miller 

 (1883 ; W. & Sp., 1897 ; Fig. LXXVIII. 4), has RR in contact except 

 on anal side ; IIIBr sometimes fixed ; iBr stouter than in Xenocrinus. 

 Both Ordovician, Ohio. Abacocrinus, Angelin (1878 ; syn. ? Carolicrinus, 

 Waag. & Jahn), Silurian, Gotland and (?) Bohemia, is more highly developed. 

 Between it and Compsocrinus we must imagine a form in which the free 

 Br became biserial, while the free rami forked several times. In Abaco- 

 crinus the proximal biserial brachials (IIIBr) with their pinnules are in- 

 corporated in the dorsal cup ; the stem has changed from sub-quadrangular 



