STRATIGRAPHICAL DISTRIBUTION. 



29 



Petrasteu. — Billing. 



Petraster rigidus. Billings. Geol. Surv. of Canada, Organic Remains, decade iii, pi. x, 

 fig. 3, p. 80. 



Description. — This species has much the aspect of an Astropeden ; the disc is one fourth 

 the whole diameter, the rays rather slender, and uniformly tapering ; the angles between 

 the bases of the rays rounded. The plates, which appear to be adambulacral, are quadrate 

 and a little convex ; 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 adam- 

 bulacral plates. The specimen figured was about two inches in diameter when perfect ; 

 width of disc, half an inch ; and of rays at the base, about three lines. 



Locality and Formation. — Trenton Limestone, Ottawa. Collected by Mr. J.Richardson. 



PALiEOCOMA,^ Salter. 



The genus Palaocoma was proposed by Mr. Salter^ to receive an interesting group 

 of .star-fishes, lately discovered at Leintwardine, Shropshire, in the thin flagstones of the 

 Lower Ludlow Rock, which were there associated with Pterygotus, Ceratiocaris of several 

 species, and Limidoides, a genus apparently allied to Limulus, together with new Crinoidea 

 and Polyzoa, and many of the more common BracJiiopoda and Graptolites of the Lower 

 Ludlow series, which are here overlain by layers of Ayraestry Limestone, full of Pentanierus 

 Knightii. 



Palaocoma is characterised by the elongated form of the narrow ambulacral ossicles, 

 which are bordered by a double row of marginal adambulacral plates, the outer row sup- 

 porting combs of long spines ; " the spines are often so long as to form a complete fringe, 

 and in one species the disc is equally spiniferous. In the curious sub-genus, Bdellacoma, 

 they are short, and intermixed with some larger clavate spines on the upper surfaces ; 

 and in Bhopalocoma, which may hereafter have to be separated as a distinct genus, the 

 hair-like spines are all absent, and clavate ones take their place."^ Palaocoma is nearly 

 related to Pteraster militaris, now living in the seas of Greenland and Spitzbergen, in 

 the spinigerous character of the adambulacral plates, but differing from it in the manner 



\ The name Palaocoma was, unfortunately, previously given by the late M. A. d'Orbigny to a genus 

 of the OphiuridcB. 



2 ' Annals and Magazine of Natural History,' 2d series, vol. xx, p. 327, pi. ix, fig. 3. 



3 Ibid., p. 327. 



