KEVISIOISr OF PALEOZOIC STELLEEOIDEA. 141 



genera and show ttie line of development from the deeply stellate 

 primary form to the pentagonal genera with well-developed inter- 

 brachial arcs. 



PETRASTER RIGIDUS (Billings). 

 Plate 27, fig. 5. 



Palasterina rigidus Billings, Geol. Surv. Canada, Rep. of Progress, 1853-1856, 



1857, p. 291. 

 Petraster rigidus Billings, Geol. Surv. Canada, Can. Org. Rem., dec. 3, 1858, 



p. 80, pi. 10, fig. Sa (not fig. 3b=I[udsonastei- matutimis). — Weight, Mon. 



British Foss. Echinod., Oolitic, vol. 2, pt. 1 (Palseontogr. Soc. for 1861), 



1862, p. 29.— Hall, Twentieth Rep. N. Y. State Cab. Nat. Hist., 1868, p. 



294; rev. ed., 1868=1870, p. 337. 



Description of 1858. — "This species has much the aspect of an 

 Astropecten; the disk is one-fom"th the whole diameter, the rays 

 rather slender and unifonnly tapering, the angles between the bases 

 of the rays rounded. The plates [of the actinal side] which appear to 

 be adambulacral [increase very little m size from the tips of the rays 

 toward the mouth], are quadrate and a little convex; [the adambula- 

 cral colmiins terminated in the mouth area by a pau' of pointed 

 oral plates and not by a single plate as shown in the original illustra- 

 tion], the marginal [inframarginal] plates oblong, and also convex 

 [certainly not less than 16 and probably 20 in each column, increasing 

 rapidly in size toward the axils, where there is a single large axillary 

 plate] ; the disk plates [accessory interbrachials] consist of three at 

 each angle [one orally and two distally], and a single row [of not more 

 than seven plates] on each side of the ray, but extending only one- 

 third or one-half of the length of the ray; they all He between the 

 [infra] margmal and adambulacral plates. [Abactinal side unknown.] 

 The specimen figured was about 2 mches [or 50 mm.] in diameter 

 when perfect ; width of disk half an inch, and of rays at the base about 

 three lines." 



Formation and locality. — Trenton hmestone, Ottawa, Canada. 

 Holotype No. 1401a is in the Victoria Memorial Museum, Ottawa. 

 The species has also been identified by Springer in the Lower Trenton 

 (Kirldiekl) at Kirkfield, Ontario. 



Remarlcs. — Hall and BiUings discussed their asterid genera and 

 species at different times and finally the former oxamuied Billings's 

 material. In this connection Hall showed that figure 36 of Petraster 

 rigidus was based on the actinal side of Hudsonaster matutinus. 

 Kegarding figure 3a, which is the holotype of this species, he in 1870 

 wrote as foUows: 



''The specimen illustrated in figure 3a has a few small intercalated 

 plates between the marginal and ambulacral [adambulacral] ranges 

 in two of the axils of the rays, and there are a smaller number of gran- 

 ules in a similar position but unequally distributed on one side of 



