R. M. Brt/doue — Further Notes on the Trinimiiujham Chalk. 293 



Here tlie front wall must have arisen by symmetrical calcification 

 of the membranous covering, starting no doubt from points along its 

 junction with the side walls and proceeding inwards. There would 

 therefore, on this hypothesis, be a morphological distinction of the 

 greatest importance in the Cribrilinidse (and through them in the 

 majority of the Clieilostomata) between cells with a distinct rim and 

 tliose without a distinct rim, a distinction which would make it 

 impossible to retain the family Cribrilinidge. It is distinctly 

 favourable to this hypothesis that Marsson has on other grounds 

 given great prominence to the question of rim or no rim in classifying 

 the Cheilostomata. The Cretaceous forms also suggest a third line 

 of development of the calcified front wall through such forms as 

 Membranipora Trimminghamensis (post). The cell in these forms is 

 clearly an ordinary Membraniporidan cell tilted forwards, and by 

 the squaring of the lower part of the aperture and pressing down- 

 wards of the straight lower lip thus formed, we pass by easy stages 

 represented by the Cretaceous forms of Pyripora to a Micropora 

 practically indistinguishable from one developed through Cribrilinid 

 forms, but with a front wall morphologically quite distinct in its 

 origin from those developed through Cribrilinid forms. 



DESCRIPTION OF SPECIES. 



A. Trimmingliam Species. 



Membranipora Griffith!,^ sp. nov. (Fig. 1.) 



Colony adherent, normally growing equally in all directions with 



almost the regularity of a Lunulites, and possessing a common crust 



out of which the cells stand sharply. Cell nearly circular, with 



a sharp-edged, narrow, smooth wall. The mature cells almost 



invariably have both an ovicell and an avicularium. Ovicell 



immediately above the cell, about half the width of the zooecium, 



Fig. 1. 



and of the usual semi-globose shape. In a large number there is 

 a triangular incision in the rounded end, apparently giving double 

 access to the interior, and the sides of the ovicell are extended along 

 the side wall of the avicularium as far as the crossbar. Avicularium 

 sharply triangular, placed immediately above the ovicell. It is 

 spanned close to its base by a very slender bar, which is often 



' Dedicated to Mr. C. Griffith, of "Winchester, a friend of long standing, who first 

 introduced me to the study of geology. 



