OF CENTRAL CANADA PART IV. 243 



III 

 BLASTOIDEA. 



This class, like that of the Cystoidea to which it is more or less 

 closely related, is entirely extinct, and chiefly characteristic of De- 

 vonian and Carboniferous strata. The typical Blastoid has an oval 

 or bud-shaped body, covered with comparatively large, regularly 

 arranged plates, with a five-rayed "pseudo-ambulacral star" at the 

 summit. The pseudo-ambulacral plates carried, it is supposed, small 

 piimulse during the life of the animal. At the underside of the body 

 there is usually a short stem. 



The known or supposed genera (for some 

 are of very doubtful position) may be ar- 

 ranged as follows : 



1. Without any inter-ambulacral aper- 

 tures : This section includes Pentremites Fig- 159> 

 (with broad-ambulacral areas, and comparatively large basal plates 

 i.e. those immediately above the stem), Upper Silurian, Devonian, 

 Carboniferous ; Granatocrinus (with long and narrow pseud-ambu- 

 lacral areas and very small basal plates) Carboniferous ; and Eleutho- 

 crinus (stemless, with four linear and one short pseud-ambulacral 

 area) Devonian. 



2. With (anal) aperture in one of the inter-ambulacral spaces : 

 Orophocrinus (with general aspect like that of Pentremites), Carbon- 

 iferous. Nucleocrinus (with very minute based plates, and long 

 pseudambulacral areas) Devonian, Carboniferous. Stephanocrinus? 

 (with long basals, and five sharp points at the summit of the calyx) 

 Upper Silurian : Niagara formation. 



Blastoids are rare among Canadian fossils, but examples of Nucleo- 

 crinus and Stephanocrinus are occasionally met with. 

 Fig. 160 shows the upper surface (about twice 

 enlarged) of Nucleocrinns Canadensis (Mont- 

 gomery), perhaps identical with N. lucina 

 (Hall), from the Hamilton formation of 



l^F Nucleocrinus 



Bosanquet township in south-western On- W Canadensis. De- 



tario.* Fig. 160 bis. is a figure of Stephan- 



ocrinus angulatus of the Niagara formation. *%*** anguiatu*. 



* A full description of this species by its discoverer, Prof. H. Montgomery, an old student 

 and graduate of Toronto University, will be found in the Canadian Naturalist, vol. x, No. 2 





