SILURIAN ANIMALS. 



307 



(Tabulate) (Fig. 307, a), giving a cellular structure which may be finer 

 or coarser. The Halysitids (Fig. 309) seem to be made up of small, hol- 

 low, flattened columns with imperfect septa, united to form reticulating 

 fluted plates, which on section have the appearance of chains crossing 

 in all directions. These are also minutely tabulated. The Syringopo- 

 roids (Fig. 308) are similar to the Halysitids, except that the hollow 

 columns are cylindrical and connect with each other only in places. 



Some of the more characteristic species of these families are given 

 above (Figs. 305-309). 



There are many other forms than those mentioned above, but their 

 affinities are little understood, and many are not true corals, but Polyzoa 

 and sponges. Nearly all the corals of Silurian, in fact, of Palaeozoic 

 times, fall under two orders — Rugosa and Tabulata. The Cyatho- 

 phylloids are Eugosa, the other families mentioned are Tabulata. The 

 Kugosa are character- 

 istic of the Palaeozoic ; 

 the Tabulata are also 

 nearly extinct : they 

 have only one family 

 living, viz., the milli- 

 pores.* The Eugosa 

 differ from modern 

 star-corals in having 

 their radiating septa 

 in multiples of four, 

 while modern star- 

 corals have theirs in 

 multiples of five or 

 six. Hence star-cor- 

 als have been divided 

 into two types — a 

 Palaeozoic and a Neo- 

 zoic — the one four- 

 parted (quadriparti- 

 ta), the other six- 

 parted (sexpartita). 

 Halysitids are charac- 

 teristic of Silurian; 

 Favositids, of Siluri- 

 an and Devonian ; Fig. mi. Fig. 312. 



and CvatllOnhvlloids FlGS - 310-312.— Living Hydkozoa: 310. Sertularia phmata: a, nat- 

 „ , " J . ural size; b. enlarged. 311. a and b, Different. Forms of Sertula- 



Of the PalCROZOic. ria. 312. Plumularia. 



Fig. 310. 



* Millipores are now shown to be Hydrozoa (Hydro-corals), 

 same may be true of Tabulata. 



It is possible that the 



