80 CANADIAN FOSSILS. 



form is deeply stellate, and the rays long and uniformly tapering. 

 A single specimen has been collected, and as it shews the under side 

 only, the characters of the dorsal surface cannot be given. The 

 structure of the mouth is also unknown. Generic name from petra, 

 a stone. 



It differs from Palasterina by the presence of large marginal plates 

 outside of the disc-plates, and still more from Stenaster, which has 

 neither disc nor marginal plates. It is allied to Astropecten. 



V. Petraster rigidus, Billings. 

 Plate X. Fig. 3. 



(^Palasterina rigidus, Geological Survey of Canada, Report, 1856, page 291.) 



Description. — This species has much the aspect of an Astropecten ; 

 the disc is one-fourth the whole diameter, the rays rather slender and 

 uniformly tapering; the angles between the bases of the rays round- 

 ed. The plates, which appear to be adambulacral, are quadrate and 

 a little couvex; the marginal plates oblong, and also convex; the 

 disc-plates consist of three at each angle, and a single row on each 

 side of the ray, but extending only one-third or one-half of the length 

 of the ray ; they all lie between the marginal and adambulacral 

 plates. 



The specimen figured was about two inches in diameter when per- 

 fect ; width of disc half-an-inch, and of rays at the base about three 

 lines. 



Explanation of Figure. Plate IX. 

 Fig. 3a. — Ventral view of an imperfect specimen. 

 Fig. 3a appears to be the dorsal side of an individual of this species with the plates 

 along the centre of the rays removed. 



Locality and Formation. — Trenton limestone, Ottawa. 

 Collector. — J. Richardson. 



Genus TiENiASTER, Billings. 



Generic Characters. — Body deeply stellate ; no disc or marginal 

 plates; rays long, slender, flexible, and covered with small spines; 

 two rows of large ambulacral pores ; adambulacral plates elongated 

 and sloping outwards so that they partly overlap each other ; 

 adambulacral ossicles contracted in the middle, dilated at each end. 

 Generic name from tainia, a riband. 



