104 CARBONIFEROUS CORALS AND BRACHIOPODS 
Portions of a thin outer wall can occasionally be. made 
out, but only in places. 
As a consequence of the extreme delicacy of the outer 
vesicular zone, it is usually destroyed more or less com- 
pletely, so that the section is bounded by the strong inner 
wall, festooned by the projections of the thick ends of the 
septa which compose the septal ring. (By this character- 
istic appearance polished sections of this coral from the 
Avon Gorge, which are common in many collections, may 
generally be easily recognized.) Very occasionally this 
inner wall is also destroyed, and nothing remains but the 
strong septal ring, rendered strikingly bilateral by the 
strongly marked fossula. 
The diameter of the inner wall seldom exceeds 3 cm. 
There are several characteristic specimens in the collec- 
tion which have been derived from a definite horizon in 
the lower portion of the Great Quarry, Avhere they are 
associated with Lithostrotion Martini. An incomplete 
horizontal section of a Caninia, derived from the Black 
Rock Quarry, and containing also sections of Zaphrentis 
(already referred to above), may belong to the mutation 
we are describing. 
This fossil must, I think, be regarded as a local mutation 
of Caninia cylindrica, with which it agrees in all the more 
important structural details ; the most striking points of 
difference in the Bristol specimens are their smaller size 
(and consequently fewer septa), and the frequency with 
which most of the outer vesicular ring has been removed. 
The horizontal section figured is the most perfect section 
I have seen ; the greater part of the outer vesicular zone is 
preserved, and the septa are conspicuously continued across 
the tabulae, almost to the centre, and exhibit the character- 
istic Zaphrentis grouping. 
? Caninia sp. 
A horizontal section shows fifty-eight strong septa 
