﻿Vol. 66. .] EVOLUTION OF ZAPHBENTIS DELANOVET. 525 



III. Paleontology. 



General Remarks. 



The gens of Zaplirentis delanouei is marked by several characters 

 that remain unchanged (except in the case of Z. lawstonensis) 

 throughout Lower Carboniferous times. These peculiarities lend 

 an unmistakable aspect to members of the group ; they may be 

 tabulated as follows: — 



(1) Average size and shape. 



(2) An epitheca with clearly marked longitudinal ribbing. 



(3) Situation of the cardinal fossula on the concave side of the corallum. 



(4) Spacing and character of the tabula?, which are arched upwards and 



strongly depressed into the fossula. 



(5) Stout septa, thickened at the inner end. 



(6) Curvature of the septa convex to the cardinal fossula. 



In my experience, immature members of the gens of Zaplirentis ennisleilleni 

 are the only forms which could be confused with the delanouei gens. In the 

 former case, however, the septa, as seen in transverse sections, are usually 

 thin, and are not thickened at the inner end ; they do not exhibit a general 

 curvature convex to the fossula ; and the fossula itself is wide and irregular in 

 outline. 



The evolutionary changes that arise are confined to the shape of 

 the cardinal fossula and the length, of the major septa ; in these 

 respects there are striking differences between typical members of 

 the mutations. 1 There is a tendency for these corals to become 

 cylindrical in their late growth-stages. In this cylindrical part, all 

 the mutations, in common with most Carboniferous Zaphrentids^ 

 usually develop short and amplexoid septa. 2 



A possible explanation of this fact may be sought in the common occurrence 

 of such fossils in soft calcareous shales. It may be conceived that during 

 growth the increasing weight of the polyp and its skeleton would sink the 

 whole organism continuously farther into the mud. A necessity was thereby 

 occasioned for some mode of growth which would raise the animal rapidly 

 above the invading sediment ; the cylindrical habit was therefore assumed. 

 In this condition growth in an upward direction would be relatively much 

 more rapid than when lateral expansion was taking place. Consequently the 

 septa might be unable to develop fully before the formation of the tabula? inter- 

 vened and stopped the further increase. 



The change does not seem to have ontogenetic significance, and 

 compels a restriction of observation to the conical part of the 

 corallum. 



Evolutionary Stages observed. 



The earliest record of the gens in Scotland is in the Cementstone 

 Group of Liddesdale, where the predominant form is' Zaplirentis 

 delanouei ; transverse sections of this species show septa meeting in 



1 Throughout this paper, unless otherwise stated, the word mutation is 

 used in Waagen's sense, and not in that of De Vries. 



2 By this term are meant septa which are only fully developed on the upper 

 surface of the tabulae, and above that position retreat rapidly towards the 

 wall. 



2o 2 



