Charles Davison—Suberial Denudation. 409 
of a decided and conclusive opinion as to the degree of relationship 
between the two. Deeper investigations are needed, and for such I 
have not at present sufficient material, especially of Heliophyllum. 
I must confine myself to stating that I observed the same structure 
as in Heliophyllum in Phillipsastrea Hennahii, Lonsd., Ph. Pengellyt, 
E. & H., Ph. Verneuilii, H. & H.! and Ph. gigas, D. Owen, but I did 
not observe the same structure in Ph. radiata, S. Woodw., and Ph. 
tuberosa, M‘Coy. 
EXPLANATION OF FULL-PAGE WOODCUT (p. 403). 
Enlarged figures showing the outline of the calices and the arrangement and the 
union of the septa. Taken from the type specimens of M’Coy in the Wood- 
wardian Museum of Cambridge. 
Fie. 1, 2, 3. Phillipsastrea radiata, 8. Woodw. sp. (=Sareinula Phillipsi, M’ Coy). 
_ Fie. 4, 5,6. Phillipsastrea tuberosa, M’Coy, sp. (=Sarcinula tuberosa, M’ Coy). 
EXPLANATION OF PLATE XII. 
Phillipsastrea radiata, 8S. Woodward sp. Carboniferous Limestone. 
Fic. 1, Specimen from Haford-y-Calch, near Corwen, North Wales. Brit. Mus. 
[R 547]. Natural size. 
Fie. 2. Specimen from Avon Section, near Bristol. Brit. Mus. [56740]. Calices 
more mammillated, approaching Ph. twherosa, M’Coy. On other parts 
of the same specimen the surface is smoother. Natural size. 
Fig. 3. Horizontal section of specimen, Brit. Mus. [R 547], showing the arrange- 
ment of septa and vesicles. Hnlarged about 64 times. 
Fic. 4. Vertical section of specimen, Brit. Mus. [R 547]. The lower part of the 
section shows the union of the septa in the central axes of the calices. 
The arrangement of tabule and vesicles is also shown. Enlarged about 
8 times. 
Fie. 5. Vertical section of specimen, Brit. Mus. [56740], showing the cone-shaped 
variety of the tabule. In other parts of the same specimen the tabule 
are more horizontal. Enlarged about 7 times. 
Fic. 6. Specimen from Avon Section, near Bristol. (My collection.) Horizontal 
section across a corallite with cone-shaped tabule, represented as rings 
in the middle part of the corallite. Hnlarged about 5 times. 
Fie. 7. Specimen from Haford-y-Calch, near Corwen, North Wales. (My col- 
lection.) Horizontal section showing in the upper corallite the rudi- 
ments of a wall, which are drawn more faintly than the similarly-shaped 
vesicles. Enlarged about 43 times. 
Fie. 8, Horizontal section of specimen, Brit. Mus. [R 547], showing the minute 
structure of the septa and vesicles. Enlarged about 50 times. 
Fie. 9. Vertical section of specimen, Brit. Mus. [R 547], showing the minute 
structure of the septa and tabule. Enlarged about 50 times. 
VII.—Nore on tHe Mwuan Rate or Suparriat Denvpation. 
By Cuarues Davison, M.A., 
Mathematical Master at King Edward’s High School, Birmingham. 
N his memoir on “ Modern Denudation,” ? Dr. A. Geikie determined 
approximately the number of years required by the following 
seven rivers to lower the general surface of their basins by one foot. 
He found that the Danube would take 6846 years, the Mississippi 
6000, the Nith 4723, the Upper Ganges 2358, the Rhone 1528, the 
1 The above-mentioned structure is very well shown in a specimen, lent me by 
Dr. Hinde, for which kindness | owe him my best thanks. 
* Glasgow Geol. Soc. Trans., vol. iii. p. 164. 
