THE GRAPTOLITES AND CORALS 303 



When a Zaphventis grows large it may pass from 

 conical to cylindrical in shape, and then a change takes 

 place in the septa : instead of continuing as vertical plates 

 through the later-formed tabulae, they retain the form seen 

 in the calyx, of low ridges on the upper surface of the 

 tabula rising up on the inner face of the theca as low 

 vertical ridges. Such a condition is described as the 

 " amplexoid habit," because it attains its fullest develop- 

 ment in the genus Amplexus. In this form, the tabulae 

 are. so horizontal and far apart, and the septa reduced to 

 such feeble ridges, that J. Sowerby, who first described 

 it a century ago, mistook it for a cephalopod-shell, and, 

 thinking the resemblance to a coral was accidental, called 

 the species he described Amplexus coralloides. The same 

 habit is seen, rather less pronounced, in the genus 

 Caninia, species of which may attain a gigantic size. 

 This is distinguished from Zaphventis by the far greater 

 abundance of dissepiments between the septa for some 

 distance in from the theca. These two genera occur, 

 like Zaphrentis, in the Carboniferous Limestone of 

 England and Belgium. 



If we now try to form an idea of the animal of which 

 the skeleton has just been described, we can only do so 

 by analogy with living corals. The main living part of 

 the body of Zaphrentis must have lain within and beyond 

 the calyx, and perhaps it could shrink up within the 

 calyx completely when safety required it. In structure 

 it was essentially a polyp, but more complex than a 

 hydroid polyp. From the tentacle-surrounded mouth a 

 short tube projected down into the general cavity, and 



