48 



must be studied in such cases by very careful comparison of 

 the impressions of the surfaces. In limestone beds tlie sub- 

 stance is often well exposed by atmospheric influences, but in 

 such instances the poriferous face seldom clearly appears, 

 owinof appai-ently to tbe former adhesion of this face to the 

 rock." This passage is especially applicable to the fossils of 

 Tasmania. In most cases we have only impressions on 

 argillaceous rocks, while the well preserved specimen now re- 

 ferred to is a limestone rock. Here but for the dark compact 

 surrounding matrix and the accompanying mass of Steno2)ora 

 ovata Lonsd. the white crystalline network of Polyzoan frag- 

 ments might almost seem to be from the Polyzoan limestone of 

 the middle Cainozoic of Australia. Butexce2:)t in few instance 

 the fossils are most pi'ovokingly nearly all face downwards. 



There are 28 known species of Penestella in British rocks 

 according to Morris' Catalogue, in which are none of the 

 Australian species. They range from the L. Silurian to the 

 Permian formation, but their princijjal horizon seems to be 

 the Devonian. Three species and a variety are known in 

 Tasmania, the species all represented in Australia and the 

 variety also probably. They are thus described : — 



Fenestella ampla Lonsdale.* Cupshaped, celluliferous, sur- 

 face internal, branches dichotomous, broad, flat, thin ; meshes 

 oval ; rows of cells numerous, rarely limited to two, alternate ; 

 transverse connecting processes sometimes cellular ; inner 

 layer of non-cellular surface very fibrous ; external layer very 

 granular, non-fibrous, gemmuliferous vessel small. 



" Among the specimens of this coral," continues Mr. 

 Lonsdale, " contained in the collection under consideration was 

 one which afforded some interesting changes dependent upon 

 age, the absence of which in the series originally examined 

 was alluded to in the species. In the uppermost portion of 

 this specimen the casts of the cellular surface exhibited 

 similar characters to those displayed in Mr. Darwin's series^ 

 with the addition occasionally of a crescent-shaped impression 

 under the mouth, and due, it is believed, to a local modifica- 

 tion of tbe sculj^turing on the surface of the other cells. A 

 little lower the ridges, or furrows representing, them began to 

 disappear, and still lower by a further thickening of the 

 exterior all traces of them were obliterated, the interspaces 

 between the mouths displaying irregular protuberances ; and 

 that which was considered as a state bordering upon decrepi- 

 tude exhibited casts of minute oral apertures, with longer 

 projections immediately beneath marking the original exten- 



• These and the two following descriptions are taken from Darwin's Geo- 

 logical Observations in South America, etc. , page 1(53. Appendix. Quoted 

 also in Strzelecki's New South Wales and Van Diemen's Laud, page 2G8. 



