'bearing on Geology and Zoogeography. 65 



districts, while the pelagic and abyssal maintained during all 

 the geological part their continuity, and are still at present 

 perfectly continuous topographically. This continuous change, 

 especially within the littoral district, during the different geo- 

 logical periods is very remarkable, l^o doubt, these changes 

 take a considerable part in developing a great diversity of 

 species. 



But not only the differences of the life-districts and their 

 topographical features effect separation of the inhabitants, but 

 within each life-district, independently from the topographical 

 continuity, separate parts are present. I refer to the differ- 

 ences of what is called facies (1. c, p. 23). Although the differ- 

 ences of facies are first recognized by geological science, and 

 although it is well known that the living as well as the fossil 

 organisms are influenced in a considerable degree by the facies, 

 and are adapted to and dependent on particular facies, this 

 factor, I think, is not yet considered properly in palaeontology. 

 In papers discussing the different faunas of the same geological 

 period, differences of the fossil remains found in strata of 

 apparently the same age, are often attributed to separation of 

 the respective sea-basins by land,* as the only means produc- 

 ing a different development of species in the seas thus sep- 

 arated : but separation by differences of facies may induce the 

 same process. Falseontologically and geologically, it is a well 

 known fact, that certain groups of animals prefer particular 

 facies, but we have no definite proof yet, that different but 

 nearly allied species are found in different facies, and in most 

 cases we do not, or only incompletely, know the particular con- 

 ditions under which the different beds were deposited. 



We see often that in the geological succession one facies is 

 replaced by another, and very often we see that the imbedded 

 fauna becomes entirely different accordingly. In other cases, 

 however, we have in an overlying, bed apparently the trans- 

 formed descendants of the underlying fauna, and lastly, I wish 

 to direct attention to a fact, which appears at first strange, 

 namely the reappearance of an older fauna in younger strata.f 

 It may be possible that such cases are due to a change of 

 facies : the original facies and its peculiar fauna are replaced 

 in a particular locality by another facies with its fauna, but 

 later on the first occupied this locality again, and the old fauna 

 reappeared. 



* I refer for example to a recent paper of Weller : A circum-insular Palaeozoic 

 fauna.— Journ. Geolog., Chicago, iii, 8, 1895, p. 903-927. I do not wish to criti- • 

 cise this very important article as regards the results obtained, but only to sug- 

 gest, that the diffiereaces of facies certainly present in the localities discussed 

 should be considered also. 



f Compare Barrande's " colonies " in the Silurian strata of Bohemia, and H. S. 

 Williams' paper entitled : The recurrence of Devonian fossils in strata of Carbon- 

 iferous age, this Journal, xlix, 1895. 



Am. Jour, Sci. — Fourth Series, Yol. II, No. 1. — July, 1896. 

 5 



