chief Generic Types of Palceozoic Corals. 425 



Upon the whole, therefore these two genera are marked off 

 from one another by characters of a more fundamental and 

 recognizable nature than those which separate Zapkrentis 

 from Cyathophyllum. 



The external form of the corallum in Amplexus is charac- 

 teristically cylindrical, though this, of course, cannot be said 

 to be distinctive of this genus. In addition, the corallum is 

 very commonly tortuous, often of very unequal thickness in 

 different parts, and frequently of great length as compared 

 with its diameter. The accretion-ridges are sometimes very 

 marked, at other times not so much so ; and the epitheca is 

 thin, and occasionally (as in A. nodulosus, Phill., PI. XII. 

 fig. 2) marked with hollow spinulosc or nodular outgrowths. 



The septa afford one of the most striking of the generic 

 characters, being invariably very short and, comparatively 

 speaking, rudimentary. This is well seen in transverse sec- 

 tions of Amplexus coralloides, Sow., and A. nodulosus, Phill. 

 (PI. XII. figs. 1 & 2). They differ in their length ; but in no 

 species of the genus do they ever extend further inwards to- 

 wards the centre of the visceral chamber than, perhaps, about 

 a third of the total diameter of the corallum, whilst in most 

 species they are much shorter than this. In typical species, 

 as in A. coralloides (PL XII. fig. 1), no secondary septa are 

 developed ; and there may or may not be intsrseptal dissepi- 

 ments, whilst these, when present, are always comparatively 

 simple and few in number. 



The tabula? are very well developed, and are usually bent 

 in a more or less marked manner at their outer extremities 

 (see the longitudinal section of A. coralloides, PI. XII. fig. In). 

 Owing to the rudimentary condition of the septa, there is 

 invariably a space in the centre of the corallum which is occu- 

 pied exclusively by the tabula?. This central space, in which 

 the tabula? alone are exposed to view, varies from about a 

 third to three fifths or more of the total area included within 

 the wall of the corallum ; and in no case which has come 

 under our observation are the septa continued over the bare 

 and smooth upper surfaces of the tabula?. The distance of the 

 tabula? from one another is variable ; but they usually extend 

 completely across the visceral chamber, unless they be much 

 crowded, in which case they may sometimes coalesce (see 

 longitudinal sections of A. coralloides, fig. 1b, and A. nodulosus, 

 fig. 3a). The tabula? are approximately fiat in the central 

 area of the corallum ; and the fossula, when recognizable at 

 all, seems to be formed by a depression in the tabula?, which 

 is occupied by a septum of smaller length than the others. 

 This is the case, at any rate, with Amplexus coralloides, though 



Ann. cb Mag. N. Hist. Ser. 4. Vol, xvi. 30 



