THE LOWER SILURIAN, PERIOD. 105 
the modern Sea-pens, Organ-pipe Corals, and Red Coral, 
rather than to the typical stony Corals. Amongst the charac- 
Fig. 45.—Columnaria alveolata, a Rugose compound coral, with imperfect septa, but 
having the corallites partitioned off into storeys by ‘‘tabulz.” Lower Silurian, Canada. 
(After Billings.) 
teristic Rugose Corals of the Lower Silurian may be mentioned 
species belonging to the genera Col/umnaria, Favistella, Strep- 
telasma, and Zaphrentis; whilst amongst the ‘“ Tabulate” 
Corals, the principal forms belong to the genera Chetretes, 
Halysites (the Chain-coral), Cozstellaria,and Heliolites. ‘These 
groups of the Corals, however, attain a greater development 
at a later period, and they will be noticed more particularly 
hereafter. 
Passing on to higher animals, we find that the class of the 
Echinodermata is represented by examples of the Star-fishes 
( Asteroidea), the Sea-lilies (Crinoidea), and the peculiar extinct 
group of the Cystideans (Cys¢oddea), with one or two of the 
Brittle-stars (OpAiurotdea)—the Sea-urchins (Echinoidea) being 
still wanting. The Crinoids, though in some places extremely 
numerous, have not the varied development that they possess 
in the Upper Silurian, in connection with which their structure 
will be more fully spoken of. In the meanwhile, it is sufficient 
to note that many of the calcareous deposits of the Lower 
Silurian are strictly entitled to the name of ‘ Crinoidal lime- 
stones,” being composed in great part of the detached joints, 
and plates, and broken stems, of these beautiful but fragile 
organisms (see fig. 12). Allied to the Crinoids are the singular 
creatures which are known as Cystideans (fig. 46). These are 
generally composed of a globular or ovate body (the “ calyx”), 
supported upon a short stalk (the ‘ column”), by which the 
organism was usually attached to some foreign body. ‘The 
body was enclosed by closely-fitting calcareous plates, accu- 
