TABULATA. 



203 



septa (typically twelve in number), are mostly present, there 

 are well-developed tabulae, and the walls are imperforate. 



In Halysites, the " Chain-coral," which may be taken as 

 the type of this family, the corallum (fig. 91), consists of 

 long tubular corallites, united to one another in such a way 

 as to form vertical plates or expansions, which, in turn, are 

 so disposed as generally to form by reticulation a loosely 

 netted mass. The tabulae are horizontal. The " Chain- 

 corals " are characteristically Upper Silurian forms, but they 

 occur also in the Lower Silurian. 



d 



Fig. 91. a, Halysites catenularia, small variety, of the natural size ; b, Fragment of a large 

 variety of the same, of the natural size ; c, Fragment of limestone with the tubes of Halysites 

 agglomerate, of the natural size ; rf, Vertical section of two tubes of the same, showing the 

 tabulae, enlarged. Niagara Limestone (Upper Silurian), Canada. (Original.) 



The genus Syringopora (figs. 92-95), is in many respects 

 closely allied to Halysites, though very different in external 

 aspect. The corallum is fasciculate, the corallites being 

 cylindrical, lengthy, and united by hollow, tubular, hori- 

 zontal connecting-processes ; so that though the wall is im- 

 perforate, the visceral chambers of contiguous corallites are 

 placed in communication. Septa, though very rudimentary, 

 are not wholly absent, and the tabulae have the form of 

 funnel-shaped plates invaginated into one another. Young 

 forms, and the basal portions of old colonies, closely resemble 



