1879.] NATURAL SCIENCES OP PHILADELPHIA. 299 



suture (which becomes oblique in ITomocrinus), in Poteriocrinus 

 it supports a third anal plate, and by pushing the right radial to 

 one side assumes its oblique position which it maintains through- 

 out all genera of the Poteriocrinus group ; while in the Cyalho- 

 cr'inus group the compound radial becomes modified into a single 

 plate. 



Geological position. Lower Silurian with the exception of a 

 single species from the Niagara group. 



The following species are known : 



1856. Dendrocrinus acutidactylus Billings. Geol. Surv. Can., p. 266; also Dec. 



iv. p. 37, pi. 3, fig. 2 a, b. Trenton litnest. Montreal, Canada. 

 *1S43. Dendrocr. alternates Hall. (Poteriocr. alternatus.) Geol. Rep. N. Y., 



vol. i. p. 83, pi. 28, figs. 1 a-f. Trenton limest. Middleville, New York. 

 *1S70. Dendrocr. angustatus Meek & Worth. (Homocr. angustatus.) Proc. 



Acad. Nat. Sci. Phil., p. 30 ; also Geol. Rep. 111., vol. vi. p. 492, pi. 23, fig. 



8. Hudson River Gr. Illinois. 

 1866. Dendrocr. caduceus Hall. (Poteriocr. (Dendrocr.) caducous.) Descr. New 



sp. Crin. etc., p. 3; also 24th Rep. N. Y. St. Cab. Nat. Hist. 1872, p. 208, 



pi. 5, figs. 7, 8; also Geol. Surv. Ohio, Pal., vol. i. p. 26, pi. 3, figs. 1 a, b, c. 



Hudson River Gr. Lebanon, Ohio. 



1871. Dendrocr. Casei Meek. (Poteriocr. (Dendrocr.) Casei.) Am. Jour. Sci., 



vol. ii. (3d ser.) p. 295; also Geol. Rep. Ohio, Pal., vol. i. p. 28, pi. 3, figs. 

 2 a, b. c. Hudson River Gr. Richmond, Ind. 



1872. Dendrocr. Cincinnatiensis Meek. (Poteriocr. (Dendrocr.) Cincinnatien- 



sis.) Proc. Acad. Nat. Sci. Phil., p. 312; also Geol. Surv. Ohij, Pal., vol. i. 



p. 20, pi. 3, figs. 5 a, b. Hudson River Gr. Cincinnati, 0. 

 1856. Dendrocr. conjugans Billings. Geol. Surv. Can., p. 268; also Decade iv. p. 



41, pi. 4, figs. 1 a, b, and 2 a, b. Trenton limest. Ottawa, Canada. 

 1859. Dendrocr. cylindricus Billings. Geol. Surv. Can., Dec. iv. p. 44, pi. 3, figs. 



8 a, b. Trenton limest. Montreal, Canada. 

 1872. Dendrocr. Dyeri Meek. (Poteriocr. (Dendrocr.) Dyeri.) Proc. Acad. Nat. 



Sci. Phil., p. 314; Geol. Surv. Ohio, Pal., vol. i. p. 24, pi. 3, figs. 3 a, b. 



Hudson Riv. Gr. Cincinnati, 0. 

 1843. Dendrocr. gracilis Hall. (Poteriocr. gracilis Hall, not McCoy.) Geol. 



Rep. N. Y., vol. i. p. 84, pi. 28, figs. 2 a-d. Trenton limest. Middleville, 



N. Y. Syn. Poteriocr. subgracilis d'Orbigny. 

 1856. Dendrocr. gregarius Billings. Geol. Surv. Can., p. 265; also Dec. iv. p. 



36, pi. 3, figs. 1 a, b, c. Trenton limest. Ottawa, Canada. 

 1856. Dendrocr. Jewetti Billings. Geol. Surv. Can., Dec. iv. p. 43. Trenton 



limest. Bay of Quinte, Canada. 

 1856. Dendrocr. humilis Billings. Geol. Surv. Can., p. 270; Dec. iv. p. 39, pi. 3, 



fig. 4. Trenton limest. Ottawa, Can. 

 1856. Dendrocr. latibrachiatus Billings. Geol. Surv. Can., p. 270 ; also Dec. iv. 



p. 39, pi. 3, figs. 5 a, b, c. Hudson River Gr. Charleston Point, Anticosti. 

 1852. Dendrocr. longidactylus Hall (Type of the genus). Geol. Rep. N. Y., vol. 



ii. p. 193, pi. 43, figs. 1 a-k ; also pi. 42, figs. 7, a, b. Niagara Gr. Lock- 

 port, N. Y. 



