246 



MINERALS AND GEOLOGY 



is open or uncovered. The mouth is in the centre of the under-side, 

 and the body-covering is chiefly coriaceous, or partly tuberculated or 

 plate-covered. The star-fishes date from the Silurian period, but 

 fossil examples in the lower rocks are comparatively rare. The class 

 is usually subdivided into two orders : Brisingida, connecting the 

 class with that of the Ophiuroidea, the arms being distinct to some 

 extent from the body cavity ; and Stellerida, comprising the true 

 star-fishes. 



The Brisingida are unknown in the fossil state. The Stellerida 

 may be conveniently arranged under three sections as follows: 

 1. Multiradiata, with more than five, usually from 13 to over 20 

 arms or rays (e.g. Lepidaster, Upper Silurian; Solaster, a living type, 

 dating from the Jurassic period; Luidia, &c.) 2. Cnrtiradiata, with 

 very short or in some cases almost suppressed rays, the shape being 

 then pentagonal (e.g. Palasterinus, Lower Silurian; Palceocoma,~Upper 

 Silurian ; Goniaster, the " cushion-stars," first appearing in Liassic 

 strata ; Asterodiscus, &c). 3. Quinquerwliata, with five well-devel- 

 oped rays (e.g. Palceaster, Stenaster, Cambrian and Lower Silurian to 

 Carboniferous ; Aaterias, Oreaster, &c.). 



The genera Palasterina, Palceaster and Stenaster are occasionally 

 represented in our Lower Silurian strata, more especially in the 

 limestones of the Trenton formation. In Palasterina, the rays ex- 

 tend a very little way beyond the central part of the body : P. Stel- 

 lata, a pentagonal form, is our best-known species. In Palceaster 



(including Petraster) the form is dis- 

 tinctly five-rayed, and the ambulacral 

 furrows are bordered by two rows of 

 small plates ; whilst in Stenaster 

 ( = Ur aster ella] , also a distinctly five* 

 rayed form, the ambulacral grooves are 

 margined by a single row. Our prin- 

 cipal species comprise, Palceaster rigi- 

 dus from the Trenton, and P. bellulus 

 from the Niagara formation ; Stenaster 



Fig. 161 bis. 



Petraster Bellulus, after Billings, 

 agara Formation. Grimsby, Ont. 



Ni- 



pulchellus, with very narrow rays, and 

 S. Salteri with comparatively broad rays, both from Trenton strata. 

 Full descriptions of these species, by the late Mr. Billings, will be 

 found in Decade III. of " Canadian Organic Remains," issued in 

 1861 by the Geological Survey. 





