BRITISH FPPER-SILTJEIAN FENESTELLID^. 245 



it is a reproduction of the cup-shaped Fenestella base, with the ap- 

 pearance of reversed fenestrules from incrusting remains, as ori- 

 ginally given by Lonsdale in the ' Silurian System.' The value of this 

 reference to D'Orbigny is considerably lessened when it is known that 

 he takes Lonsdale's Fenestella prisca for the type of his new species, 

 offering the very doubtful advantage of a change in the nomencla- 

 ture only. The same mode of treatment is adopted in the case of 

 Fenestella antiqua, Lonsd., which became the type of D'Orbigny's 

 Fenestella subantiqua. I have mentioned that most of Lonsdale's 

 drawings of Fenestella j^risca are disfigured by the enveloping coral 

 to which I have alluded. On the other hand, Fenestella Milleri, 

 Lonsd., is covered with a species of Aulopora. In practice, I find 

 that in our museums it is usual to assign the Fenestella base, covered 

 as in Fenestella prisca, to Fenestella Milleri. It is not a matter of 

 much moment, since the identity of this particular species is past 

 recovery ; the probabilities of the case are in favour of their being 

 the same species. This growth on Fenestella piisca is readily de- 

 tected by the peculiar shape of the so-called fenestrules, which 

 are often twice as broad as long. What I take to be the tabulae 

 of the coral occur more frequently than the dissepiments of the 

 Fenestella ; hence the alteration in the shape of the fenestrules. 

 This in itself is quite a reversal of the usual order of growth, and 

 a feature as yet unknown in the Fenestellidee. Prof. JN'icholson, 

 indeed, mentions an instance (Fenestella Jilifonnis^', from the Devo- 

 nian of America) in which the order of the fenestrules is thus re- 

 versed. The identification of this species as a Fenestella 1 consider 

 to be reasonably open to doubt, since the author states that the pori- 

 ferous face is unknown, thus leaving the true character undeter- 

 mined. Can it be that the poriferous face is covered up with some 

 foreign growth, as in the Silurian forms ? It seems strange that, 

 as in this case, the attempt should be made to describe a new species 

 of Polyzoa without having seen the cells or celluliferous face. I may 

 remark here that this is not the first time in which the incrustation 

 on Fenestella has proved a source of error to palaeontologists. Some- 

 thing akin to this led Prof. M'Coy to found the genus Hemitrypa, 

 which in a former paper I showed to be Fenestella memhranacea 

 (Phil.), incrusted by a minute coral or polyzoonf. This is from 

 the Carboniferous Limestone. Phillips mentions a similar form in 

 the Devonian Limestone. 



Five years after arranging the Silurian Polyzoa we find Lonsdale 

 suggesting the parasitic nature of the genus Hemitrypa, in reference 

 to some fossils from "Van Dieraen's LandJ. He makes no allusion 

 whatever to the occurrence of the same growth among the Silurian 

 group of Polyzoa. 



Penestell.v reticulata, Lonsd., Murch. Sil. Syst. p. 678, pi. 15. 

 fig. 19. 

 The specimen in the Museum of the Geological Society is a frag- 



* Geol. Mag. 1874, p. 199. t Quart. Jouru. Geol. Soc, toI. xxxt. p. 282. 



:j: Darwin's 'Volcanic Islands' (1844), p. 163. 



