176 TRILOBITES OF GIRVAN. 



being almost absent on entering the Silurian period, when the general features of 

 the common British fauna overspread the area. 



Geographical Distribution. — The recognition of more or less distinct zoo- 

 geographical marine provinces in Europe in the distribution of trilobites during 

 Lower Palaeozoic times is mainly due to Freeh ; l and though recent work has 

 rendered some modification necessary in his lists of peculiar genera or subgenera 

 supposed to be characteristic of the several provinces, yet the main principles may 

 be regarded as well established. The existence, in fact, of four distinct provinces 

 is supported by an ever-increasing amount of evidence. To what extent these 

 geographical divisions hold good with respect to other zoological groups of 

 organisms during the same period remains largely still uninvestigated. 



The British Isles belong, according to Freeh's scheme, to the North Atlantic 

 marine province, the Baltic and Bohemian-Mediterranean provinces being distinct. 

 The invasion of England by Baltic forms has been referred to by me on a former 

 occasion,- but the genus Nieszhowskia has not been mentioned previously in this 

 connection. The contingent from the Bohemian province in the Whitehouse Beds 

 is of much interest (apart from Arethusina JconincJci?, which occurs in the 

 Balclatchie Beds) ; it appears and disappears with equal suddenness. 



The relation of the trilobitic fauna of the Pomeroy and Waterford areas to that 

 of the Girvan district is deserving of notice. In the case of the Waterford area 

 the presence of a Scandinavian element has been pointed out; 3 and there is a 

 resemblance, though faint, in the Waterford and Girvan trilobites, as shown by 

 the occurrence of the rare genus Apatokephalus (= Tramoria) and of allied or com- 

 parable species, apart from widely distributed British species, such as Phacops 

 brongniarti. The Drummuck Beds in the Girvan area contain several species 

 identical with those of Keisley and Kildare (e.g. Liclias bulbiceps, Remopleurides 

 colbii, Phillipsinella parabola, etc.), but with the possible exception of the first 

 mentioned, all these species occur elsewhere. The Scandinavian element at 

 Keisley has been elsewhere referred to as especially strong, but it is less prominent 

 at Drummuck. The characteristic species of this horizon of the Bala series have a 

 remarkably wide horizontal distribution. 



But on the whole the communication between the different basins of deposition 

 in Ordovician times in Europe, and even in the British Isles, seems to have varied 

 irregularly in degree and in duration. The determining factors in the migrations, 

 counter-migrations, and interchange of faunas were undoubtedly physical to a 

 large extent, but we must not forget that biological influences, the importance of 

 whicli it is at present impossible to estimate when dealing with fossil faunas, were 

 probably also at work, and may ultimately help to explain the many puzzling 



1 Freeh, 'Leth. Palaeoz.,' vol. ii (1897), pp. 88—95. 



2 Reed, ' Quart. Journ. Geol. Soc.,' vol. liii (1897), pp. 96-100. 

 :i Reetl, 'Quart. Journ. Geol. Soe.,' vol. lv (1899), pp. 768—770. 



