S2 CANADIAN FOSSILS. 



This species is larger and more robust than the former. Both 

 appear to be somewhat common, and the specimens are often found 

 with their rays variously curved, showing that they were extremely 

 flexible. 



Explanation of Figures. Plate X. 

 Figure 4a. — Dorsal view of a specimen. 



4b. — Ventral view of a different specimen. 



Locality and Formation. — City of Ottawa. Trenton limestone. 

 Collector. — E. Billings. 



Genus Edrioaster, Billings. 



(Genus Cydaster, Geological Survey of Canada, Report, 1856, page 292.) 



Generic Characters. — Body sessile, circular, discoid, covered with 

 numerous irregularly polygonal plates ; mouth large, sub-pentago- 

 nal ; five ambulacral grooves, each composed of two series of oblong 

 ossicles ; four rows of ambulacral pores to each groove. The mouth 

 is formed of five oral and apparently five internal ossicles. The 

 sutures between the ambulacral ossicles, in certain conditions of pre- 

 servation, are enlarged so that the pores on each side are connected, 

 and thus there appear to be two instead of four. Hence in the first 

 description I gave of this species it was described as having two 

 rows. Since then more perfect specimens have been procured, and 

 I am enabled to correct this misconception. Generic name from 

 edriao, to seat, in allusion to the sessile condition of the species. 



In my report for 1856 this genus is called Cyclnster ; but I find 

 that this name had been a short time previously given to a genus of 

 sea-urchins by M. Cotteau, and it is therefore necessary now to pro- 

 vide a new one. (See Catalogue des Echinides Fossiles des Pyrennees. 

 Par MM. Leymerie et Cotteau. Bulletin de la Soci^t^ Greologique 

 de France, 18 Fevrier a 17 Mars 1856.) This number of the Bulle- 

 tin was published in March 1857, but my Report was not issued until 

 the autumn following. 



VIII. Edrioaster Bigsbyi, Billings. 



{Cyclaster Bigsbyi, Geological Survey of Canada, Report, 1856, page 293.) 



Description. — The body of this species is circular, about one inch 

 and a half across, and half an inch in height in the centre. It is 

 covered with numerous small plates of various sizes, and, except in 

 the ambulacral areas, disposed without order. The mouth, situated 



