﻿280 DE. A. VATJGHAN ON THE PALjEONTOLOGICAL [May I905, 



The typical member of this group, Lithostrotion Portlocki (Bronn), 

 Edwards & Haime, differs from L. basaltiforme in (1) its smaller 

 size : (2) its very strong columella ; and (3) the common tendency 

 to Clisiophyllidan and Zowsdafo'a-modifications. 



Mutations of Lithostrotion, 



Nothing can be stated with certainty concerning the evolution of 

 this group. The earliest form with which I am acquainted occurs 

 near the top of the Syringothyris-Zone ; it resembles Lithostrotion 

 irregulare in its smaller size, in the strength of its columella, in 

 the fewness of the septa, and in the conical tabulae, but the inner 

 and outer walls are never coalescent, and the septa do not extend 

 to the columella ; it also differs in its ramulose manner of growth. 



The commonest forms of the two sections that are found in the 

 fieminula-Zorie are Liihostrotion Martini and L. basaltiforme. 

 L. affine and L. aff. Portlocki make their appearance in S 2 . 



The Dibunoj>hyllum-Zone is characterized by both Konincko- 

 phyllidan and Clisiophyllidan modifications of the Siphonodendron- 

 section, and by both Clisiophyllidan and Lonsdalia-like modifications 

 of the JSTematophyllum-section, as represented by members of the 

 group of Lithostrotion Portlocki, 



The Clisiophyllids. 



In order to explain the terms employed in the explanatory notes 

 which follow, it is necessary to describe, in detail, the horizontal 

 section of a typical Clisiophyllid. 



Such a section exhibits four areas : central, medial, external, and 

 peripheral. 



The central area is occupied by a system of curvilinear intersections, the 

 fundamental plan of their arrangement being that of a spider's web. 



The medial area is radiated only by the primary septa, and has few dissepi- 

 ments in the interseptal spaces. 



The external area is radiated by the primary septa and, usually, by a less- 

 developed secondary series. The interseptal spaces are crowded with 

 dissepiments which, by their closer approximation at the inner margin of 

 the area, form the inner wall. 



The peripheral area, at its inner boundary, usually merges, quite insensibly, 

 into the external area ; it is distinguished by the predominance of vesicular 

 over radial structure. 



Septa : — 



The primary septa are, usually, very thin and inconspicuous in the peripheral 

 area, but much thicker in the external and medial areas, the maximum 

 thickness being attained at about the inner wall. 



The secondary septa are always very thin, and seldom extend inwards as far as 

 the inner wall ; they are often practically absent, or but partly developed. 



The various genera of this family which have been created by 

 Thomson, Thomson & Nicholson, and Thomson & Duncan, are 

 described and illustrated in Proc. Phil. Soc. Glasgow, vol. xiv (1883) 

 pp. 296-502, pis. i-xiv. 1 



1 This paper is a summary of numerous earlier papers which appeared in the 

 same Proceedings. 



