196 THE GENUS MONTTCULIPORA. 



at intervals of from a line to a line and a half, and are oc- 

 cupied by callces which are not conspicuously larger or smaller 

 than the average. The walls of the corallites are at first thin ; 

 but they rapidly become thickened, the lines of demarcation 

 between adjacent tubes still remaining recognisable. A few 

 remote, complete, and horizontal tabulae are developed in 

 the interior of the tubes ; but these are not recognisable in all 

 the corallites, and are often placed at corresponding levels in 

 adjoining tubes. 



Obs. — This beautiful form presents a considerable superficial 

 resemblance to Ptilodictya, and has been referred to this genus. 

 It wants, however, the definitely circumscribed and peculiarly 

 marked lateral margins of the fronds of this Polyzoan type, 

 and, what is more important, it is without the peculiarly 

 striated central lamina of the Ptilodictycc. It is true that the 

 bases of the corallites in M. pavonia, D'Orb., are so united 

 with one another as to orive rise to an irreijular calcareous 

 membrane, which separates the two halves of the corallum ; 

 but none of the specimens that I have seen exhibit any 

 tendency to split along the line of this membrane, nor can the 

 corallites be forcibly removed from one side of it, exposing 

 the median lamina as a definite structure. In both these re- 

 spects the Ptilodictyce would show quite different phenomena. 

 The reasons just given would equally prevent my accepting 

 the view held by Mr E. O. Ulrich that this form should be 

 referred to Heterodictya, Nich., since this genus differs from 

 Ptilodictya principally in the possession of tabulae. On 

 the other hand, the general aspect of AI. pavonia, D'Orb., is 

 not at all remarkably different from that of the thin fronds 

 of M. frondosa, D'Orb., while it exhibits low " monticules ; " 

 and there is nothing in its internal structure which is irre- 

 concilable with the view that it is a Mojtticulipora. In the 

 meanwhile, therefore, I shall leave this type within the limits 

 of the genus Moiiticuliporay till more definite evidence may be 

 forthcoming as to its proper position. 



So far as external characters go, M. pavonia, D'Orb., is 



