Reviem — Br. G. Lindstrom's Silurian Gasteropoda, etc. 37 



graph contains the results of his researches on the Gasteropoda and 

 Pteropoda from the Silurian strata of that locality, so famous for the 

 number and perfect condition of its fossils. 



The author gives a preliminary sketch (accompanied by an outline 

 map) of the strata and the different localities in which the fossils 

 occur, and indicates the physical conditions which probably prevailed 

 at the time the Mollusca flourished. It appears that the lowest beds 

 in Gotland are shales, occasionally replaced by sandstones, from 20 

 to 100 feet in thickness. The lowest zone of these shales contains 

 StricJclandinia lirata, Euomphalus Gotlandiciis, Pleiirotomaria qiial- 

 teriata, var., and a species of Eai-pes, and it corresponds with the 

 Upper Llandovery of this country ; all the beds above this zone are 

 of Wenlock age, though they occasionally contain fossils which have 

 been thought to characterize Upper Ludlow rocks. In the lowest 

 division, 65 species of Gasteropoda are present, 29 of which are 

 limited to it, whilst 36 pass upwards to the beds above. These con- 

 sist of thin flaggy limestones, with seams of coarse marly shales, in 

 all about 70 feet in thickness, which occur in Northern and Central 

 Gotland ; but in the South of the Island they are replaced by a 

 coarse pisolitic or oolitic rock, not more than 20 feet in thickness. 

 From this division 64 species are known, but only 12 ai'e peculiar to 

 it. Succeeding this division are beds of so-called crinoidal limestone, 

 at least 150 feet in thickness, which cover a wide area in the Northern 

 and Central portions of Gotland. In some places this limestone is 

 almost exclusively composed of crinoidal remains, whilst in others, 

 either Corals, Brachiopods, Lamellibranchs, Gasteropods, or Cepha- 

 lopods predominate to the exclusion of other forms. This upper- 

 most division of the strata of the island is by far the richest in 

 Gasteropoda, having 124: species, of which 81 are peculiar to it. 



The state of preservation of the shells is, of course, variable in the 

 different strata, but it presents some peculiarities not readily ac- 

 counted for. Thus in the shales of one locality only the casts or 

 moulds of the shells are preserved, whilst in the same beds but a 

 short distance apart, the shells themselves are retained. Again, 

 whilst some genera are only represented by casts, others inter- 

 mingled with them in the same beds and locality possess perfect 

 shells, which probably indicates original differences in the chemical 

 or mineralogical constitution of the different genera.^ The shells, when 

 present, are now mostly of a crystalline calcite, but in some the inti- 

 mate structure can be recognized, and in certain examples even the 

 pattern of the original colour-bands and traces of the nacreous lustre. 



Out of a total fauna of 1007 species in the Silurian strata of 

 Gotland, the Gasteropoda and Pteropoda number 174 species, belong- 

 ing to 25 genera. But a very small proportion of these has been 

 recognized beyond the limits of Gotland, and only the following 13 

 species are common with the Silurian strata of this country, viz. 

 Flatyceras eornutum, His., Platy. spiratum, Sow., Flevroioniaria linmta 

 {=:Eaomphalus carinatus, Sow.), P. labrosa, Hall, P. alaia, His., 



1 See H. C. Sorby, F.R.S., "On Structure of Shells," Anniversary Address to 

 Geol. Soc. Lond. 1879, pp. 56-93, Quart. Joui-n. Geol, Soc. vol. xxxv. 



