GENERAL REMARKS. 175 



more than the definitely determined peculiar Girvan species. There have been 

 described in the preceding memoir 135 distinct species and several varieties; and 

 even if we cut out those which cannot be identified or compared with described forms 

 from any region, we still are left with less than one third of the whole number 

 of species which are known from other localities in the British Isles. The local 

 Girvan element has been shown to form nearly as large a proportion of the whole. 

 Moreover, it cannot fail to be noticed that, considering the relative abundance of 

 species in the Ordovician and Silurian beds, the majority of the typical British 

 forms do not appear till Silurian times. Out of the total number of twenty-seven 

 Silurian species recorded in the Girvan area there are nineteen British species, as 

 the above list shows ; while out of the total eighty-three determinable Ordovician 

 species of Girvan there are only about twenty-five occurring elsewhere in the 

 British Isles, and many of these are rare or occur typically only in Ireland. The 

 proportion of the British element is therefore much larger in Silurian times than it 

 was in Ordovician times, and seems to indicate a sudden inrush of species from the 

 south at the beginning of the Llandovery period by the breaking down or removal 

 of some intervening barrier. 



Foreign Elements. — The two elements of the trilobitic fauna which have been 

 above mentioned are not the only ones which can be isolated. There is a distinct 

 Bohemian element represented by the following species: Agnostus perrugatus, 

 A. tardus, Telephus fr actus, Gyclojpyge anuata, G. rediviva, Bohemilla, sp., Arethusina 

 koninclci. With the exception of the last-named species they are confined to one 

 horizon — the Whitehouse Group. 



The Baltic or North European element is more widely distributed, ranging 

 through the whole Ordovician, and is represented not so much by identical species 

 as by allied species or by others belonging to some of its peculiar genera 

 NieszJcowsJcia, Sphserocoryjphe, Youngia, etc. We may enumerate the following 

 forms : 



Ampyx depressus ? Lichas (Conolichas) cf. xquiloba, 



„ ci.foveolatus, Acidaspis dalecarlica, 



cf. scanicus, Cybele cf. aspera, 

 Apatukephalus sp. (also iu S. Ireland), ,, belled ula ? 



lie mupleur ides cf. nanus, Cheirurus (Nieszlcowslcia) unlcus, 

 Illxnus cf. oculosus, „ (Sphxrocvryphe) thomsoni, 



Proetus cf. obconicus, „ {Youngia) trispinosus. 



Lichas aff. marainatns, 



Individuality of the Fauna. — In spite of the various constituent elements of the 

 trilobitic fauna, its individuality is strongly marked by the number of its peculiar 

 species and the presence of two peculiar genera. The individual character which 

 it thus possesses is most conspicuous during Ordovician times, the local element 



