298 PROCEEDINGS OF THE ACADEMY OF [1879. 



1856. Hybocr. lumidus Billings. Geol. Surv. Canada, p. 275; also Decade IV. p. 



28, pi. 2, figs. 1 a-e. Trenton limestone. Ottawa, Canada. 

 1859. Hybocr. pristinus Billings. Geol. Surv. Canada, Decade IV. p. 23, pi. 1, 



fie. 2 a. Chazy limestone. Montreal, Canada. 



5. DENDROCRINUS Hall. 



Diagram PI. 16, Fig. 5.) 



1852. Dendrocrinus Hall. Geol. Rep. N. Y., vol. ii. p. 193. 

 1859. Dendrocrinus Billings. Geol. Surv. Canada. Decade IV. p. 85. 

 1873. Dendrocrinus (subgenus of Poteriocrinus) Meek. Geol. Surv. Ohio, 

 Pal., vol. i. p. 20. 



A. Typical form. 



General form of the crinoid elongate and slender. Calyx ob- 

 conical, higher than wide, unsymmetrical. 



Underbasals five, similar in form, scarcely of medium size, but 

 extending beyond the column. Basals five, the largest plates in 

 the calyx; four of them equal, hexagonal, the fifth or posterior 

 one hep t agonal, truncate above for the support of a large anal 

 plate. Radials alternating with the basals all around, simple in 

 four of the rays, pentagonal and of about equal size. The right 

 posterior radial is compound, divided b}' a horizontal suture into 

 two halves, which, taken together, have about the form of the 

 simple plates, only slightly longer. Brachials two to five, some 

 long and narrow, and others short and wide. Anals one, sub- 

 quadrangular. 



Arms long, branching; ambulacral furrow deep. Pinnulse 

 "wanting. 



Dome unknown. Ventral sac strongly developed, composed of 

 numerous small, hexagonal, alternately interlocking plates of 

 equal size, strengthened by little transverse or slightly oblique 

 costfle, and so arranged as to present an ascending zigzag appear- 

 ance. 



Column pentagonal, or exceptionally round. 



The compound plate in the right posterior ray is one of the best 

 characters of Dendrocrinus, and distinguishes it readily from Gya- 

 thocrinus with which it is most likely to be confounded. The 

 lower h.ilf of the radial has nearly the same position and form as 

 the first oblique anal plate in Poteriocrinus, and both these plates 

 rest against the sloping side of the posterior basal, with an anal 

 plate on the left ; but while this plate in Dendrocrinus is succeeded 

 by the upper segment a radial plate separated by a horizontal 



