THE LOWER SILURIAN PERIOD. IO5 



the modern Sea-pens, Organ-pipe Corals, and Red Coral, 

 rather than to the typical stony Corals. Amongst the charac- 



Fig. 45. Columnaria alveolate, a Rugose compound coral, with imperfect septa, but 

 having the corallites partitioned off into storeys by " tabulae." Lower Silurian, Canada. 

 (After Billings.) 



teristic Rugose Corals of the Lower Silurian maybe mentioned 

 species belonging to the genera Coliimnaria, Favistella, Strep- 

 telasma, and Zaphrentis ; whilst amongst the " Tabulate " 

 Corals, the principal forms belong to the genera Chcetetes, 

 Halysites (the Chain-coral), Constellaria, 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 

 Echinoderinata is represented by example^ of the Star-fishes 

 (Asteroidea), the Sea-lilies (Crinoidea), and the peculiar extinct 

 group of the Cystideans (Cystoidea), with one or two of the 

 Brittle-stars ( Ophiuroidea) 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- 



