494 NORTH AMERICAN INDEX FOSSILS. 



Artificial Key to the Genera.^ 



A. Arms free above the radials I. 



I. Disk composed of only five orals in the form of a pyramid. Ambulacra subteg- 



rainal ( Larviformia ) i . 



I. Radials simple a. 



a. Upper edge of radials bevelled off to form straight articular facets. 



IV. Symbathocrinus. 



a. Upper edge of radials deeply forked, the prongs extending upward be- 

 tween the arms .II, Stephanocrinus. 



1. One or three of the radials compound. Calyx small b. 



b. Three of the radials compound III. Haplocrinm. 



b. The right posterior radial compound I. Pisocrinus. 



I. Posterior side of disk extended into a sac. Ambulacra suprategminal [Fislulata 



and Artjculata) 2. 



2. Monocyclic c. 



c. Ventral sac small, extending but little above the rest of the tegmen ii. 



II. Arms simple V. Hybocrinus. 



II. Arms branching... ., ..,.aa. 



aa. Two radials compound. ^ VI. Anomalocrinus. 



aa. Three radials compound VII. Heterocrinus. 



aa. No radials compound ; arms extremely broad. 



XXIV. Edrioc7'inus. 



c. Ventral sac large. Arms three 22. 



22. Ventral sac touching the two simple radials VIII. Calceocrinus. 



22. Ventral sac not touching the two simple radials... IX. Halysiocrmus. 

 2. Dicyclic d. 



d. Infrabasals rudimentary or completely resorbed. Joints of column in the 

 form of 5-angled stars LXXVIII. Pentacrinus. 



d. Infrabasals and basals present though often hidden 33 



33. All radials simple bb. 



bb. Ventral sac coiled Xll. Botryocrimis. 



bb. Ventral sac not coiled j. 



|. Arms without pinnules ..■^. 



*. Arms rather delicate, \^^ 



1" . Anal plate i XIII. Cyathocrinus. 



I^^. Anal plates 3 .X. Carabocrinus. 



■^. Arms massive XIV. Barycrinus. 



1. Arms with pinnules ^*. 



**. Articular facets of radials crescentic and rarely occupying 



the full width of the plates 2''. 



2^\ Arms branching XV. Poieriocrinus. 



2^^, Arms simple XXIII. Agassizocrinus. 



**. Articular facets of radials straight and completely occupied 



by the lower edge of the first brachials '}/' . 



y^. Dorsal cup deep XVII. Scytalocrinus. 



2,^^. Dorsal cup low a^'' . 



a.^^. Arms strongly zigzag., XVIII. Decadocrinus. 



2/'. Arms not strongly zigzag \''^. 



i^^^. Arms branching XVI. Scaphiocrinus. 



Adapted in the main from Wachsmuth and Springer. 



