﻿Vol. 66.] EVOLUTION OF ZAPHBEXTIS DELANOUEI. 527 



After a considerable vertical interval, the gens undoubtedly 

 reappears in the Lower Limestone Group (Carboniferous Limestone 

 Series), and in that position can be found all over Southern Scotland, 

 developed in a highly characteristic manner. 



Since the Lower Limestones lie some 2000 feet above the coral- 

 bearing beds of the Liddesdale Cementstones, it is in accordance 

 with expectation that in the Group as a whole there is but a very 

 small percentage of adult forms belonging to the parent species, 

 Zaplirentis delanouei. The parallela mutation, also, has fared little 

 better. A new mutation, the presence of which is foreshadowed in 

 the Cementstone assemblages, now becomes predominant, and in its 

 ontogeny presents features of considerable interest. 



Serial sections of a typical representative of this new mutation 

 show that, in the neanic stage, the fossula retains little or no trace 

 of the typical delanouei character. But the parallel-sided, though 

 still relatively wide, outline, found in adults of Zaphrentis parallela, 

 is clearly visible (PL XXXVII, fig. 5 d). As growth proceeds, the 

 inner end of the fossula narrows (PI. XXXVII, figs. 5 b & 5 c) ; and 

 in sections across the ephebic region the constriction becomes pro- 

 nounced (PI. XXXVII, fig. 5«) : the septa are still joined together 

 in the centre of the corallum. On account of the fossular character, 

 this mutation may be termed Zaphrentis constricta. 



Although within the Lower Limestone Group Z. constricta is 

 almost invariably the predominant member of the gens, forms 

 representing a further advance in evolution are also present in 

 some force. These new forms, as a rule, do not pass through a 

 parallela stage in neanic life, but develop instead the constricta 

 type of fossula (PI. XXXVII, figs. 6 e & 7 e). On further growth 

 the septa shorten, until in ephebic life they become amplexoid in 

 character, and separate at the centre of the corallum (PI. XXXVII, 

 figs. 6 a, 7 a, & 7 b). The amplexoid septa lend a very distinctive 

 appearance to this mutation, which may accordingly be termed 

 Zaphrentis disjuncta. 



It is worthy of note that the less advanced forms of Z. disjuncta 

 frequently present, in their fossular character, a great resemblance 

 to Z. parallela (PI. XXXVII, figs. 6 b & 6 c), due to the shortening 

 of the septa in the cardinal quadrant being insufficient to cause 

 a separation of their inner ends. Xo confusion with the older 

 mutation need arise, if the striking difference between the neanic 

 stages of the two forms be considered (compare PI. XXXVII, 

 figs. 6 e k 4 d). 



Passing on to the Upper Limestone Group, it is found that 

 Zaphrentis delanouei is now altogether absent, Z. parallela extremely 

 rare, and that Z. constricta, although by no means uncommon, is 

 quite subordinate to Z. disjuncta, which becomes easily the dominant 

 form. Many specimens of the last-named species, especially in the 

 upper part of the Group, present a further advance in type, the 



