﻿74 ME. T. r. SIJiLT ox THE PATJNi.L SUCCESSION IN THE [Feb. 1 908^ 



The closely-set primary septa are thickened in the medial area, 

 thin and flexuous in the external area, and extend to the outer wall. 

 Their inner ends are attached to the mesh of the central area. The 

 secondary septa, which are regularly developed, are thin and flexuous 

 in the external area ; their thickened ends project a short distance 

 inwards from the inner wall. The septal fossula is conspicuous. 



The external area, radiated by both series of septa, is filled with 

 rectangular dissepiments; and the inner wall is defined by a 

 thickening of the innermost dissepiments. 



Discussion. — This coral, which occurs somewhat rarely in tlie 

 upper part of D^ in the Midland area, is essentially identical with 

 Clisiophyllum M' Coyanum, Thomson, as described and figured in 

 Proc.Phil. Soc. Glasgow, vol. xiii (1880-82) p. 526 & pi. v, figs. 4-46. 

 In Thomson's figures, however, the central area is more closely^ 

 reticulate than in the Midland specimens. 



Bibunophyllum, Thorns. & Nich.^ 



DiBTJNOPHYLLTTM MATLOCKENSE, Sp. HOV. (PI. I, fig. 2.) 



Description. — The form is elongate-conical. 



In a horizontal section : — The central area is completely, or 

 nearly completely, bisected by a strong mesial plate. The mesial 

 plate, which is of practically -uniform thickness throughout its 

 length, always extends to the circumference of the area on the 

 fossular side, but sometimes falls short of the circumference, by a 

 short distance, on the opposite side. In some sections, the mesial 

 plate is seen to be produced into the fossula, and to become 

 attached to the primary septum occupying the fossula ; while in 

 some other sections it bends round at the end, and becomes attached 

 to one of the septa bounding the fossula. The radial lines, re- 

 presenting the cut edges of the vertical lamellae, are well-spaced, 

 strong, and fairly straight. 



The primary septa are appreciably thickened in the medial area, 

 thin and flexuous in the external area, and extend to the outer 

 wall. Thin secondary septa are generally well-developed in the 

 external area, but they rarely reach the inner wall, and never 

 extend inside it. The septal fossula is always conspicuous. 



The external area, of moderate width, is uniformly and rather 

 closely vesicular. An irregular thickening of the dissepiments 

 round the inner margin of the external area gives rise to a poorly - 

 defined inner wall. 



Horizon. — This species occurs in the Loiisdalia-snhzone of the 

 Midland area, and is there the predominant Dihunopliylliim. 



Variation. — The forms which I include in DihunophyUum 

 matlockense show marked individual variation as regards the develop- 

 ment of a secondary series of septa, and the number of radial 

 lamellae in the central area. The figured type has secondary septa 



^ This genus is fully described by Thomson & Nicholson, Ann. & Mag. Nat. 

 Hist. ser. 4, vol. xvii (1870) pp. 457-59. 



