VINE : ^IICRO-PAL.^ONTOLOGY. 



211 



Survey) from one or two localities small, microscopic, perforated 

 wheel-like organisms, which he provisionally referred to the 

 Holothuroids, but very little is known otherwise than that given 

 above of these minute remains. 



GASTEROPODA. 



I have four specimens of Gasteropoda from these Shales. They 

 are very rare, and what I have belong to two Genera, Loxonema 

 Phillips, and Macrocheilus (?) PhiUips. They are not in a good state 

 of preservation, but in thus drawing attention to their presence in these 

 lower Shales better specimens might in time be found. In the Scotch 

 Shales Mr. John Young says : ' The Gasteropoda are more abundant 

 in those which alternate with the limestone than in the limestone 

 itself.' Some species are very common, others are rare. And again 

 (Trans. Edin. Geol. Soc, i88i-2,p. 85-6), 'The Scottish Carboniferous 

 univalve moUusca are as a rule considerably smaller in size than 

 species of the same genera found in the limestone strata of England, 

 Ireland, and other countries where similar strata prevail. This 

 shows that some dwarfing influence was at work ; and it has likewise 

 been noticed amongst the other groups of the mollusca. The causes 

 which led to this dwarfing may be difiicult to explain, but probably 

 it was due to less favourable conditions of the sea bottom on which 

 they lived, brought about by temperature, depth of water, its fresh- 

 ness or saltness, or by the nature of the sediments that were bemg 

 deposited.' The same rule would apply to other groups of fossils as 

 well as to the mollusca ; and every indication of the probable causes 

 of the multiplication of, or degeneracy of species, will help to clear up 

 some few of the mysteries which surround the life histories of peculiar 

 fossils. With regard to our positive knowledge of the actual 

 distributions of species, it may be noted ' that on certain horizons of 

 strata in both the lower and upper limestone series there is an 

 associated group of shells which always appear in certain of the 

 Limestone Shales and clay ironstones, and sometimes in the calmy 

 and cement limestones.' Among the bivalves Area and Cypricardia, 

 among the univalves several genera of Gasteropoda such as 

 Pleu7-otomaria, Macrocheilus, Loxofiema, &c. ' In the strata in which 

 the foregoing groups occur, remains of Corals, Crinoids, Polyzoa, 

 and the greater number of species of the Brachiopod shells are 

 absent, or at least very rare ; while on the other hand, in the strata 

 where this latter group is abundant, the foregoing group of mollusca 

 are rarely met with ' (see Cat. of Western Scottish Fossils). It is no 

 difficult matter, then, to know the difference by the association of 



April 1885. 



