458 Mr. J. Thomson and Dr. H. A. Nicholson on the 



shoAV an intermediate area (" interlocular area ") of irregu- 

 larly vesicular tabulge, and a central area of anastomosing 

 concave tabulae, which are on the whole directed upwards, and 

 are intersected by several incomplete columellarian lines, or 

 rarely by one such line. Transverse sections show that the 

 central area is divided into two equal portions by a median 

 lamina which extends completely across it, and one extremity 

 of which points to a well-developed septal fossula. 



In the general features of their internal structure the forms 

 which we have grouped together under the name of Dibuno- 

 2)hylluni present a close resemblance to those which properly 

 belong to ClisiopliyUum ; but they are nevertheless separated 

 by characters sufficiently distinct and easily recognizable to 

 waiTant their being placed in a section by themselves. The 

 species of Dihunophyllum agree with those of C Usiophyllmn 

 in their triareal structure, as shown in longitudinal sections. 

 They both possess an outer area of numerous minute vesicles 

 arranged in oblique rows pointing ujiwards and outwards, an 

 intermediate area formed by loosely and irregularly anasto- 

 mosing tabulfe, and a central area formed by vesicular tabulae, 

 which, though more or less strongly elevated centrally, are at 

 the same time concave, and are intersected by a series of ver- 

 tical lamellae. In both groups, again, we find an essentially 

 similar arrangement and a like development of the septa — 

 those of the first cycle, extending from the wall to near the 

 outer margins of the central area, having the form of well- 

 developed laminjB internally, but becoming more delicate as 

 they pass outwards, and more numerously intersected by the 

 angular dissepiments ; whilst those of the second cycle are 

 very short, and may be wanting altogether. 



With these points of resemblance there are the following 

 differences to be noted in the structure of the forms re- 

 spectively referable to Dihunopliyllum and ClisiopliyUum : — 

 (1) The central area in both genera is formed by the intersec- 

 tion of a system of vesicular tabular with a system of vertical 

 lamellae. In Dihunopliyllum this area is seen in transverse 

 sections (PI. XXV. figs. 1, 2, 3 a, 5) to be divided into two 

 equal halves by a complete mesial septum, no such structure 

 existing in Clisiophyllum proper ; vdiilst the lines represent- 

 ing the cut edges of the vertical laminas are much more irre- 

 gular in the former than in the latter, and are devoid of 

 any spiral bending. (2) One extremity of the mesial septum 

 just spoken of as dividing the central area in Dihunophyllum 

 is invariably directed towards a well-marked septal fossula 

 (PI. XXV. fig. 1). (3) Longitudinal sections show that the 



