﻿284 DK. A. VAUGHAN ON THE PAL^ONTOLOGICAL [May 1905, 



Dibunophyllum, sp., Thomson, pi. xii, fig. 3. 



Here the central area is reduced, and the inner wall more strongly marked 

 than in our species. This may be regarded as an intermediate stage between 

 Dibunophyllum 8 and Dibunophyllum 1//. 



Dibunophyllum 0. (PI. XXIV, fig. 1 a.) 



Proc. Bristol Nat. Soc. n. s. vol. x (1903) pi. i, fig. 6 {Clisiophyllum 

 turbinatum). 



The form is cylindrical, of small cross-section. It may be regarded 

 as a variant of Dibunophyllum 6 ; it differs in the narrowness of 

 the external area and, usually, in the presence of a secondary series 

 of short, thick septa. 



Clisiophyllum oblongum, Thomson, Proc. Phil. Soo. Glasgow, 

 vol. xiv (1883) pi. xiii, fig. 2, appears to be a true Dibunophyllum, 

 and to be closely related to our form. 



Dibunophyllum ^. (PI. XXIV, figs. 2 & 2 a.) 



This is a typical representative of the most highly-specialized 

 members of this section. The form is short and conical. The 

 calicinal boss has the typical characters of the genus. 



Horizontal section. — The central area has a well-defined 

 boundary, which projects, as a cusp, into the fossular gap. The 

 mesial plate is strongly developed, and continued into the fossular 

 gap. 



The primary septa taper at both ends. The secondary septa are 

 not prominent, although usually developed in a part, at least, of 

 the external area. 



The external area is crowded with vesicles and bounded by a 

 well-marked inner wall, which is formed by the very close ap- 

 proximation of two or three rings of vesicles. The peripheral area 

 is only partly developed. 



Resemblances and differences. — Clisiophyllum turbinatum, 

 M'Coy, ' Brit. Palaeoz. Foss.' 1851, pp. 88 & 96, figs, a, b, c, appears 

 to be a true Dibunophyllum, but to differ from our species in the 

 ratios of the areas and in a larger conical angle. 



Clisiophyllum turbinatum (M'Coy), Edwards & Haime, appears 

 to include our form, as well as other conical Clisiophyllids which I 

 regard as essentially distinct. From Nunney, near Frome (included 

 in the list of localities from which the specimens, examined by 

 Edwards & Haime, are derived), I only know a short, conical Clisio- 

 phyllid which differs entirely from Dibunophyllum \p in having a 

 Zaphrentid-type of structure (see below). 



Clisiophyllum turbinatum (M'Coy), Thomson, Proc. Phil. Soc. 

 Glasgow, vol. xiii (1882) p. 537 & pi. vi, fig. 6, differs considerably 

 from our form : — 



(1) Though a mesial crest is mentioned in the description, there seems to 



be no trace of a mesial plate in the figure. 



(2) The inner wall in Thomson's figure is not nearly so strongly de- 



veloped as in our form. 



