﻿52S MR. E. G. CARETJTHEES OKT THE ' [Nov. I9IO, 



septa becoming amplexoid even in the neanic stage (PI. XXXVII, 

 fig. 8 c) and being extremely so in the ephebic period (PI. XXXVI E, 

 figs. 8 a & 8b). When the gens is last seen, not far above the base 

 of the Millstone Grit, these advanced types of Z. disjuncta are 

 abundant, but no further change in ontogeny can be recorded. In 

 the Millstone Grit forms there is an absence of spines or other 

 characters usually taken to indicate phylogenetic old age. Unless 

 such a state be indicated by the rudimentary condition of the 

 septa, the extinction of the gens at this position (above which no 

 corals of any kind are recorded in Scotland) must be ascribed solely 

 to the incoming of unfavourable physical conditions. 



General Conclusions. 



Reviewing the evolution as a whole, one is struck by the 

 prevalence of tachy genesis. It rarely happens that the ancestral 

 traits seen in the neanic stage go back farther than the immediately 

 preceding form. Indeed, in a few cases, the adult fossular character 

 may be assumed in the neanic stage, and persist unchanged to the 

 close of the ephebic period. ' Skipping of stages ' l is remarkably 

 rare. 



The development of amplexoid septa in Z. disjuncta may indicate 

 catagenesis (or simplification of characters). It is, of course, 

 impossible to ascertain the raison d'etre of the progressive 

 changes observed, since the soft parts of the animal are missing. 

 In cutting sections, however, it is noticed that there is a very 

 appreciable gain in structural strength when the constricta type of 

 fossula is assumed. One learns by painful experience how the 

 large open fossula of Zaphrentis delanouei weakens the corallum. 



Little can be said yet regarding the line of descent of Z. delanouei 

 itself. Among the earliest specimens of that species (for instance, 

 those from Hartsgarth in Liddesdale, and from horizon /3 in the Avon 

 Gorge) the neanic cardinal fossula is sometimes constricted at the 

 inner end. It is possible, therefore, that the Devonian ancestor of 

 the gens was of the constricta type, and that the fossula of the 

 Visean Z. constricta illustrates the return of an old character. 2 



Determination of Specimens. 



The difference between these mutations is obviously not great, 

 and, as a matter of fact, so intimate is their relationship, that any 

 considerable assemblage will always contain a number of forms the 

 classification of which is no easy matter. In these cases the problem 



1 Recently termed saltative palingenesis by S. S. Buckman, in his 

 'Yorkshire Type Ammonites' pt. i (1909) Introduction, p. vii. 



2 A Devonian ancestor of the gens may perhaps be found in Zaphrentis 

 guillieri, Barrois ('Recherches sur les Terrains Anciens des Asturies & de la 

 Galice' Mem. Soc. Geol. Nord, .vol. ii, No. 1, 1882, p. 197 & pi. vii, fig. 3), 

 which has many characters of the gens, and in which the cardinal fossula is 

 decidedly constricted ; I have not yet been able, however, to examine specimens 

 of this interesting species. 



