1879.] NATURAL SCIENCES OF PHILADELPHIA. 293 



and forming a prominent ridge, which gives the appearance of an 

 arm. 



The ventral sac in this genus is but imperfectly known, but it 

 is apparently not so robust as in Iocrinus. Column more or less 

 pentagonal. 



The genus Heterocrinus is known exclusively from the Lower 

 Silurian, and has been found only in America, unless Myelodacty- 

 lus (?) heterocrinus Angl., from the Upper Silurian, belongs to it. 



The following species have been discovered : 



1859. Heterocrinus articulosus Billings. Geol. Rep. Canada, Dec. iv. p. 51, pi. 



4, fig. 8. Trenton limest. Ottawa, Cannda. 

 1866. Heterocr. constrictus Hall. 24th Rep. N. Y. State Cab. Nat. Hist., p. 210; 



also Geol. Surv. Ohio, Palseont., vol. i. p. 3, pi. i. figs. 10 a, b. Hudson 



River Gr. Cincinnati, 0. 

 H. constrictus (var.) contractus Meek, 1S73. Geol. Surv. Ohio, vol. i. p. 



4, pi. i. fig. 11. Ibid. 



1866. Heterocr. exilis Hall. 24th Rep. N. Y State Cab. Nat. Hist., p. 213, pi. 5, 



fig. 16. Hudson River Gr. Cincinnati, 0. 

 H. exilis (var.) exiguus Meek, 1873. Geol. Surv. Ohio, Pal., vol. i. p. 5, 



pi. 1, fig. 12. Ibid. 

 1843. Heterocr. heterodactylus Hall. (Type of the genus.) Geol. Rep. N. Y., 



vol. i. pi. 76, fig. 11 a-o ; also Geol. Surv. Ohio, Pal., vol. i. p. 12, pi. i. 



figs. 1 a, b (2 a, b ? ). Hudson River Gr. Ohio and New York. 

 1859. Heterocr. inaequalis Billings, Geol. Rep. Canada, Dec. iv. p. 51, pi. 4, fig. 



7. Trenton limest. Ottawa, Canada. 

 1866. Heterocr. juvenis Hall. 24th Rep. N. Y. State Cab. Nat. Hist., p. 212, pi. 



5, figs. 9, 10 ; also Geol. Rep. Ohio, Pal., vol. i. pi. i. figs. 3 a-c. Hudson 

 River Gr. Cincinnati, O. 



1866. Heterocr. laxus Hall. 24th Rep. N. Y. State Cab. Nat. Hist., p. 211, pi. 5, 



fi^. 15 ; also Geol. Rep. Ohio, Pal., vol. i. p. 14, pi. i. figs. 8 a, b. Hudson 



River Gr. Cincinnati, O. 

 1843. Heterocr. simplex Hall. Geol. Rep. N. Y., vol i. p. 280, pi. 76, figs. 2 a-dj 



also Geol. Surv. Ohio, Pal., vol. i. p. 7, pi. i. figs. 4, 5 (6, 7?). Hudson 



River Gr. Cincinnati, O. 

 Syn. Heterocr. Canadensis Billings, 1859. Geol. Rep. Canada, Dec. iv. p. 



48, pi. 4, fig. 5. 

 1856. Heterocr. tenuis Billings, Geol Rep. Canada, p. 273; also Dec. iv. p. 50, 



pi. 4, figs. 6 a, b. Trenton limestone. Ottawa and Montreal, Canada. 



2 IOCRINUS Hall. 

 (Diagram PI. 16, Fig. 3.) 



1866. Desc. New Spec. Crin. by J. Hall, p. 5 ; reissued in 1872 in the 24th 

 Rep. N. Y. State Cab. Nat. Hist., p. 210. 



Prof. Hall, in re-describing Heterocrinus subcrassus Meek & 

 Worth, as Heterocr. (?) pohjxo, made use of the name Iocrinus 



