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G. W. SHETJBSOLE ON THE VARIOUS SPECIES OF 



here requisite. The altered appearance presented by a worn-down 

 specimen of Fenestella from the Dudley Limestone is very great ; 

 this will necessitate certain allowances when the species under exa- 

 mination are, as is usually the case, much weathered and denuded. 

 The cell-apertures, in good specimens, are small and round, and quite 

 their diameter apart. When worn down, the cell-aperture is large, 

 and the intermediate spaces between the cells nearly obliterated or 

 they have only the body-wall between them. Again, in the more 

 perfect examples the obverse face of the interstice is full and rounded, 

 with a prominent keel ; when worn down, the sides of the interstice 

 have a sharp angular face — a great contrast to what it had been. In 

 the one case the fenestrules may be narrow originally, and become 

 altered to wide by erosion. So, too, with the reverse face ; in 

 Fenestella-gvo^th. the lines of construction give rise to strong 

 striations on the reverse side of the polyzoary, which, however, in 

 the Silurian species, are seldom seen, as the structure is generally 

 worn down and smooth. 



Certain features would be constant in the Fenestellidae (I allude 

 to the keel, the small circular pore, and the striations on the reverse) 

 were it not that they had been worn away from a variety of causes. 

 It may be interesting here to compare polyzoal life, so far as 

 concerns the Penestellidae, at the two great horizons of its growth — 

 the Silurian and Carboniferous, the early and late Palaeozoic. The 

 Lower Silurian forms have special interest, on account of their 

 apparent relationship with the Graptolites, which they seem to re- 

 place in the higher series. Unfortunately the material available 

 for comparison is so scanty, and the special organs so imperfectly 

 preserved, that the affinities between the genera cannot well be 

 ascertained, at least for the present. No such reasons, however, 

 exist to hinder the comparison between the species found in the 

 Tipper Silurian Limestone and those of the Carboniferous Limestone. 

 The result of the comparison is that we discover that the Silurian 

 .forms have a distinct yjxaes of their own, being often more minute 

 in structural details and altogether different in the general outline 

 of their growth, at the same time having the true and marked 

 features of Fenestella-^YOW^. To indicate some of the peculiarities 

 of the Silurian Eenestellidae, I may mention the strong conical 

 structure, often solid and slightly bulbous at the extreme point of 

 the base, of Fenestella lineata ; this would seem to indicate a species . 

 capable of being firmly attached by its base, and adapted to resist 

 the action of powerful currents of water. A glance at Lonsdale's 

 drawings of the Fenestellidae will show how frequently they were ' 

 possessed of this strong basal attachment ; we have nothing, strictly 

 speaking, among the Carboniferous forms to compare with it, with 

 one partial exception — the Fenestella memhranacea (Phil.). So it 

 comes to this, that the strong conical base of the Silurian Fenestella 

 is now only partially seen in one of the Carboniferous species. -, The 

 leading type in the polyzoary of the Carboniferous Penestellidae is 

 a flat expansion, slightly depressed in the centre, which formed its 

 point of attachment ; while numerous rootlets from various parts of 



