On the chief Generic Types of Palceozoic Corals. 123 



XVI. — Contributions to the Study of the chief Generic 

 Types of the Palceozoic Corals. By James Thomson, 

 F.Gr.S., and H. Alleyne Nicholson, M.D., D.Sc, 

 F.R.S.E., Professor of Natural History in the University 

 of St. Andrews. 



[Continued from p. 70.] 



[Plate Vni.] 



Genus Diphyphyllum. 



Diphyphylhim, Lonsdale, Murch. Vem. & Keys. Russ. and Ur. p. 623. 



Gen. char. Corallum compound — formed of long, slender, 

 cylindrical corallites, which are usually placed at some little 

 distance from one another, and are associated into fasciculate 

 masses. The corallites have a distinct epitheca ; and the mode 

 of increase is by parietal or calicular gemmation. The septa 

 are well developed, but do not reach the centre of the visceral 

 chamber. Internally there is a small central area, occupied 

 exclusively by the tabtdse. Externally the interseptal loculi 

 are rendered vesicular by the development of dissepiments. 

 No columella. 



The typical corals of the genus Diphyphyllum form fasci- 

 culate masses, often of very large size, composed of cylindrical 

 corallites, which may be occasionally united at certain points 

 here and there, but are usually quite free. As rightly pointed 

 out by Prof, de Koninck, Mr. Lonsdale was in error in assert- 

 ing that the increase of the corallum was effected by means of 

 fission of the old tubes. On the contrary, the increase is 

 almost always by means of parietal or calicular gemmation, 

 the old corallites continuing to .grow, and the new corallites 

 maintaining a position nearly parallel with that of the parents 

 (PI. VIII. fig. 1 a). It seems not improbable that, in one 

 species, at any rate (viz. D. Archiaci^ Billings), the corallum 

 is occasionally simple ; but this point requires further in- 

 vestigation. 



The central area of each of the corallites is occupied by 

 the tabulae, which are perfectly smooth (PI. VIII. fig. 1). 

 The floor of the cylindrical, moderately deep calice of each 

 is formed by the uppermost tabula in the centre. The septa 

 do not encroach upon the median tabulate area ; but we are 

 unable to confirm M. de Koninck's view that this area is 

 (as a rule, at any rate) surrounded by a special mural in- 

 vestment. 



There are no traces of any columella ; nor are there any 

 grounds for believing that the absence of this organ is acci- 

 dental, as formerly held by Milne-Edwards and Haime (Pol. 



9* 



