108 PEOF. J. LOGAN LOBLEY, F.G.S., F.E.G.S., ON 



the relatively short existence that the small thickness of the 

 strata they characterise may seem to indicate. They may have 

 migrated to, and lived on in, other areas at a greater or less 

 distance from that we have been able to examine. Although 

 the theory or principle of homotaxis as propounded by Huxley 

 cannot be allowed to apply to the extent its author anticipated, 

 it yet has undoubtedly a considerable kernel of truth, for 

 migration may entirely remove a species from a locality and 

 give it to another where it will be contemporaneous with later 

 deposits. 



The difference in the fossil fauna of the same formation in 

 two localities not very far apart is remarkable. If we take the 

 Inferior Oolite of Gloucestershire and Dorsetshire, for example, 

 we find an abundance of Brachiopoda in the former and an 

 abundance of Cephalopoda in the latter. The Terehratula 

 suhglohaia is most numerous near Stroud and almost absent in 

 Dorsetshire, while almost only at Crewkerne in Somersetshire 

 is Ceromya Bajoceana to be found. From one small locality in 

 Dorsetshire a large number of species of Ammonites have been 

 obtained, while in other localities the Inferior Oolite gives only 

 a few of these species. ^sTear Enslow Bridge, in Oxfordshire, 

 the Great Oolite contains a bed in which Terchrahda maxillata 

 is most abundant, but any such a congeries of this species is 

 not to be found elsewhere. Yet in none of these cases is a 

 species altogether confined to one locality, and as more and 

 more places are examined the evidence of wider extension is 

 obtained. In two Austrian areas of contemporaneous Triassic 

 rocks it has been recently ascertained that the fossils of one are 

 very different from the fossils of the other, and that some 

 remarkable zones with Palaeozoic species are only to be found 

 in one of these areas. 



The very small area in which a number of individuals of a 

 species may be localised, as it were, in a colony, is strikingly 

 shown by the occurrence of that fine gasteropod the Purpuroidea 

 Morrissia. Thirty or forty years ago this fossil was abundant 

 in the Great Oolite of Minchinhampton, while now it is not to 

 be found there. The same bed is exposed but the continued 

 working of the quarry has removed a few horizontal yards of 

 rock which has obliterated the little colony, but only a colony, 

 since it is not to be concluded that no other individual of this 

 species lived in otlier areas on this geological horizon. At the 

 present time there are thousands of cockles on our coasts in certain 

 places and not a single cockle in others, even where the condi- 

 tions are similar ; and so it is with mussels, periwinkles, etc. 



