164 R. G. Ccirruthers — A Revision of some Carboniferous Corals. 



3. The (cardinal) fossula has no continuous ^all (as in Zaphrentis 

 honincki, q.v.). The bounding septa are often unconnected, and the 

 inner end is not closed by septa, but by the intersection of a tabula 

 in the plane of section. In sections across the amplexoid part of 

 the coral the size of the fossula varies considerablj^ according as the 

 section is cut across the top or bottom of the tabular depression. 



4. Tlie rudimentary minor septa, appearing in the more mature 

 part of the coral, never transgress the delicate dissepimental ring 

 there found. Where they seem to do so, it will be found that the 

 supposed ' inner wall ' of dissepiments is in reality the epitheca 

 repeated by rejuvenescence. One, or two, rings of dissepiments are 

 seen in average specimens ; in unusually broad examples, however, 

 three or even four rings may be present, but such cases are rare. 



(b) Vertical Sections. 



PI. VI, Figs. \h and 1/, and Diag. P, Figs. 2 and 3. 



These show that the tabulae are arranged in a fairly regular manner, 

 varying from "5 to 2 mm., but on the whole about 1 mm. apart. They 

 are dome-shaped in the lower or conical part of the coral, but become 

 flattened in the cylindrical poi'tions. Their depression into the cardinal 

 fossula is at tirst very marked,' but lessens somewliat in the cylindrical 

 portions (PI. VI, Pig. \h, and Diag. F, Figs. 2 and 3). The tabular 

 do not extend through the thin marginal zone of dissepiments developed 

 in the adult stages of growth. These dissepiments have an elongated 

 outline, and are directed upwards and outwards at a very steep angle. 

 They are usually thin and delicate, there being rarely more than two 

 rows of them. They appear to be additional structures, and in no 

 way caused by a '* splitting of the wall." 



Summary. 



There are several constants of value in the determination of this 

 species. They are: — (1) The epithecal characters; (2) the spacing 

 of the major septa (i.e. their number in a given diameter); (3) the 

 thickening of stereoplasma in the lower and conical part; (4) the 

 characters of the dumonti phase (PI. VI, Figs, \h-ld and 2a-2d) ^ 



(5) the nature of the dissepiments, and their restriction to the mature 

 growth stages ; (6) the nature of the tabular. Of these, (2), (4), and 



(6) are of particular importance. 



Localities (both from Dr. Vaughan). 



Tournaisian (subzone 7) : Burrington Coombe (Mendips) ; Frome 

 (lowest exposure in the Vallis Vale sequence of quarries). 



^ la many vertical sections cut dowu the cardinal fossula, some of the tabulse, 

 while strongly depressed, are seen to bend upwards before reaching the waU. This is 

 due to the fact that the fossular depression, instead of lying wholly in the plane of 

 section, enters it obliquely at those points, and at some distance from the wall. Such 

 an appearance will always be noticed if the coral be twisted so that a straight plane: 

 of section cannot always pass down the centre of the fossula. 



