OF CENTRAL CANADA PART IV. 



245 



rian, Trenton Formation, but are com. 

 paratively rare. The genus Edrioaster 

 of Billings only differs from Agelacrinites 

 in shewing pores in the am bulacroid spaces; 

 buo pores (obliterated by fossilization) were 

 probably present originally in all forms. 

 Figure. 161 represents an example of Hemi- 

 cystites Billingsii from the Trenton Lime- 

 stone of Peterborough in Ontario, Other 

 species, but commonly of smaller size, 

 occur in the same formation at Ottawa and elsewhere. 



Fig. 161. 



Hemicystites (Agelacrinites) 



Billingsii ; Chapman. Trenton 



Formation. 



V. 



OPHIUROIDEA. 



This class comprises the so-called " Brittle Stars," or star-fishes 

 with typically small central body, and five long, thin, more or less 

 serpentiform arms or rays, entirely distinct from the central stomach- 

 cavity, and without open ambulacral groove. Both disc and arms 

 are generally plate-covered, but in some forms they are coriaceous 

 only, or partially tuberculated. The month is in the centre of the 

 underside of the disc, and there is no separate anal orifice. 



Two orders are generally recognized : Euryalida, with coriaceous 

 integument, and mostly with branching arms which are capable of 

 being curled towards the mouth ; and Ophiurida, the true brittle 

 stars, with plate-covered disc and arms. Examples of both orders 

 date from the Silurian period ; and some few genera (Protaster, Eugas- 

 ter, &c.) are exclusively Palaeozoic. Tceniaster cylindricus (Billings), 

 with narrow arms and partially overlapping, spinous plates, from 

 the Trenton Formation, is our best known representative. 



VI. 



A5TEROIDEA. 



The representatives of this class comprise the starfishes proper, in 

 which the stomach cavity is continued into the so-called arms or rays. 

 In most, there is an anal orifice ; and, in all, the ambulacral groove 





