BRITISH UPPER SILURIAN FENESTELLIDJ5. 253 



the structure, but more especially from near the base, served to secure 

 it in position, -which would seem to have been the sides of projecting 

 rocks or suitable exposures. Of these non-poriferous rootlets, 

 which are quite a feature in the Carboniferous Penestellidae, and 

 notably so in FenesteUa plebeia^ M'Coy, we have no trace whatever 

 in the Silurian species. We may notice the beginning of the Car- 

 boniferous type of polyzoarium in the wide cup-like expansion of 

 one of Lonsdale's forms, Retepora infundibulum. The resem- 

 blance here is all the more complete, since the poriferous face is on 

 the inside of the polyzoary, as in the Carboniferous species. Again, 

 we may compare the position of the poriferous face. In FenestcUa 

 Jineata, a prominent and leading species among the Silurians, it is 

 on the outside of the polyzoarium ; in all the Carboniferous species it 

 is on the inner side of the same. So, too, we may notice that the pre- 

 vaiHng Silurian species are all much smaller than the Carboniferous. 

 Our knowledge of the cell-character in the Silurian Fenestellce is 

 necessarily limited ; but this we may say, that as yet no material 

 difference has been noticed, either as to cell-structure or internal 

 arrangement, as compared with the Carboniferous. The angular 

 knife-edged character of the interstice in Silurian species, often 

 alluded to by Hall* and Lonsdale f as a distinguishing mark, is the 

 result of denuding agencies; the original condition may often le 

 seen by careful searching in some fold or protected part of the poly- 

 zoarium, when it will be seen that the obverse face was full and 

 rounded, with projecting circular pores, similar to the Carboniferous 

 types. 



There are, besides what I have mentioned, several interesting in- 

 dividual features which may be noticed. For instance, in the lower 

 horizon of the Bala beds Retepora is abundant and Fenestella scarce ; 

 while in the Dudley Limestone Fenestella is abundant and Retepora 

 absent, so far as I am aware. The interesting point here is that 

 some of the Fenestellce in the Dudley beds, in which as yet we have 

 no record of Retepora having been found, have strong affinities to 

 Retepora or, more strictly speaking, Polypora^ M'Coy; for the genus 

 Polypora is really the connecting-link between Retepora and 

 Fenestella, combining, as it does, many of the features of either 

 group. It is not too much to say of Fenestella intermedia that 

 there is on the polyzoary an alternate interchange of polyzoal 

 features ; that on a branch may be seen for a short distance the 

 double row of cells divided by the customary keel, succeeded by 

 three rows of cells and no keel strictly speaking ; this interchange 

 of character is persistent over the expansion. Thus we have here 

 typical Fenestella and typical Polypora, not only on one branch, but 

 on the whole — an alternation of character, Fenestella and Polypora^ 

 that, to say the least, is very interesting, as showing the connexion 

 between Retepora and Fenestella. I have not met with any like 

 features among the Carboniferous species, and therefore conclude 

 that they are confined to the Silurian types. 



Another species having evident affinities with Retepora, even more 

 * Hall's Pal. New York, vol. ii. p. 50. t Murcli. Sil. Sjst. p. 678. 



