REVISION OF PALEOZOIC STELLEROIDEA. 139 



Inframarginal plates bounding the entire outer edge of the animal, 

 conspicuous, closely adjoining, not stellate and common to both the 

 actinal and abactinal areas. 



Madreporite radially striate, abactinal. 



Adambulacrals closely adjoining the inframarginals in the distal 

 portion of the rays but proximally the former are separated from the 

 latter by more or less numerous, small, accessory interbrachial plates. 

 The adambulacrals may be of nearly the same size, or considerably 

 wider than long in the mid-length of the column. The adambula- 

 crals of adjoining columns meet orally in a pair of larger triangular 

 plates, the oral armature. 



Interbrachial areas more or less large and the space between the 

 inframarginal and adambulacral columns filled with a variable 

 number of accessory interbrachial plates. 



Ambulacrals slightly alternating, about as numerous as the adam- 

 bulacrals, each with an L-shaped, proximally converging carina. 

 Podial openings through the outer proximal corner of the plate and 

 proximal to the carina. Grooves narrow, regularly tapering. 



Genoholotijpe (type by monotypy). — Petraster rigidus Billings. The 

 above diagnosis is based on the genotype and Palseastenna speciosa 

 Miller and Dyer. 



Distribution. — Ordovicic of North America, Silmic of Australia. 



The species are: 



P. rigidus (Billings). Trenton. 



P. speciosus (Miller and Dyer). Maysville, Richmond. 



P. (?) americanus (D'Orbigny). Maysville. 



P. srmjtJii McCoy. Siluric of Australia. 



Remarks. — While Billings has correctly pointed out the diagnostic 

 characters of Petraster, yet the genus has been very imperfectly 

 understood. This may partly be accounted for in that he later also 

 included a specimen of Hudsonaster matutinus (Hall), which of course 

 fails to have the essential generic character of Petraster, namely, the 

 interbrachial accessory plates. The following is Billings' original 

 description: 



''This genus has both marginal and adambulacral plates, with a 

 few disk-plates on the ventral side. The general form is deeply 

 stellate, and the rays long and uniformly tapering. A single specimen 

 has been collected, and as it shows the underside only, the characters 

 of the dorsal surface can not be given. The structm-e 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 disk-plates [that genus has smaU marginals], 

 and still more from Stenaster, which has neither disk nor marginal 

 plates. It is allied to Astropecten.^' 



