﻿Vol. 6 1.] SEQUENCE IN THE BKISTOL AKEA. 273 



The general characters of this species have been sufficiently 

 described by previous writers, but the following notes are intro- 

 duced to explain the mutational stages. 



The central portion of the calyx, at an adult stage of growth, is a 

 broad, flat tabula, surrounded by a circular, moat-like groove ; the 

 wall of the calyx is very thick, and its inner face slopes down to 

 the groove. 



The septa are numerous, and are strongly developed in the region 

 of the groove ; from this region they stretch inwards, along the face 

 of the tabula, and outwards, along the sloping face of the wall. 

 It is, as a rule, only in the immediate neighbourhood of the groove 

 that the prolongations of the septa are sufficiently tall to reach from 

 tabula to tabula, or from one layer of the vesicles, which form the 

 inner surface of the wall, to the next layer. Hence, the characters 

 of a horizontal section are as follows : — 



The outermost vesicles of the thick wall usually show no septal 

 projections, but these projections become longer and stronger as we 

 pass inwards. 



The septa usually are strongly thickened in the region of the 

 groove, though not uniformly so all round the whole circumference, 

 the septa in the neighbourhood of the fossula being always the 

 most strongly thickened. This phenomenon is probably connected 

 with the fact that the tabula? slope down to the fossula. 



The septa are cut off sharply where the plane of section cuts 

 across a tabula. The interruptions of the septa, and the abrupt 

 change from a thick septum to a thin one, are due to the fact that 

 the plane of section, after cutting through one tabula, intersects the 

 septa which lie on the next lower tabula. 



Mutational Forms. 

 Mutation y . (PI. XXIII, fig. 1.) 

 This form is characterized by the slight extension of the septa 

 over the vesicular wall, so that, in a horizontal section, the septa 

 are only represented by short projections from the vesicles in 

 this region. 



Mutation S r (PL XXIII, fig. la.) 



Oaninia cylindrica, var. bristolensis, Vaughan, Proc. Bristol Nat. Soc. 

 n. s. vol. x (1903) p. 103 & pi. i, fig. 4. 



Here, the septal prolongations are more' strongly developed over 

 the vesicular wall, so that, in a horizontal section, the external 

 vesicular area is delicately radiated by continuous prolongations 

 of the septa. There is also a greater development of vesicles in the 

 interseptal spaces. 



In both these characters there is an approach towards a Cyatho- 

 phyllidan structure, but the strong fossular depression, and the 

 marked thickening of the septa in the middle of their length, 

 separate this form as a true Caninia. 



