OF CENTRAL CANADA PART IV. 



239 



All the Articulata, on the other hand, are Post-palseozoic types, fossil 

 genera being found especially in Triassic, Jurassic, and Cretaceous 

 formations ; and about eight living genera are known.* 



Nmerous examples of crinoid stems in a more or less fragmentary 



condition occur in our Silurian and 

 Devonian rocks. Figure 153 repre- 

 sents a piece of shale, from the vicinity 

 of Toronto, covered with portions of 

 Crinoid stems, some seen in transverse 



FIG. 153. 



Crinoid stem-fragments. section, and others longitudinally ; and 



similar examples are abundant in the Chazy, Trenton, Niagara, and 

 Corniferous limestones, of other parts of Central Canada. Well pre- 

 served or entire examples of crinoids, and especially of the calyx (on 

 which, distinctive characters, as regards genera and species, are chiefly 

 based) are, on the other hand, comparatively rare. The stems, un- 

 fortunately, are of little use in the determination of genera, as the 

 character of the stem differs frequently in different species of the 

 same genus ; and occasionally the same stem is found tv> vary in 

 different parts of its own length. Next to the stems, fragments of 

 crinoid arms are of most frequent occurrence. In the following 

 enumeration, therefore, of some of our more commonly occurring 

 forms, the genera are arranged after the more easily recognised arm- 

 characters : 



1. Arms with pinnulce, 



Glyptocrinus : Pinnulse very fine ; arm-plates in single row ; 

 calyx-plates radiately ridged ; stem generally round, 

 the plates alternating in diameter, (sometimes pen- 

 tangular) ; stem-orifice, five angled. Figure 154. 



Thysanocrinus : arms long and thin, bifurca- 

 ting ; arm plates in a double row, otherwise much 

 like Glyptocrinus : Silurian and Devonian, 



Dendrocrinus : arms thin, long, much branch- 

 ing ; calyx-plates, large ; stem five-angled. Lower 

 Silurian. 



Heterocrinus : arms long, simple or bifercat- 

 ing, with strong pinnulse ; arm plates in single 

 row ; stem variable. Lower Silurian. 



In 1882, the Author proposed a new classification of the Crinoids in three groups and twenty- 



