io6 HISTORICAL PALEONTOLOGY. 



partitions, which divide the coral into so many tiers or storeys, 

 and which are known as the " tabulae " (fig. 45). 



In addition to the Rugose Corals, the Lower Silurian rocks 

 contain a number of curious compound corals, the tubes 

 of which have either no septa at all or merely rudimentary 

 ones, but which have the transverse partitions or " tabulae " 

 very highly developed. These are known as the Tabulate 



Fig. 5.Columnaria alveolata, a Rugose compound coral, with Imperfect septa, but 

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

 Canada. (After Billings . ) 



Corals; and recent researches on some of their existing allies 

 (such as Helioporajhave shown that they are really allied to 

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

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

 teristic Rugose Corals of the Lower Silurian may be mentioned 

 species belonging to the genera Columnaria, Favistella, Strep- 

 telasma, and Zaphrentis; whilst amongst the " Tabulate " 

 Corals, the principal forms belong to the genera Chatetes, 

 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 

 Echinodermata is represented by examples 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 



