GENERAL REMARKS. 177 



sudden appearances and disappearances of the foreign species in British areas and 

 the seemingly sporadic distribution of certain types. 



Vertical Range of Genera. — Apart from the general characters of the whole 

 assemblage of trilobites in the Girvan area we may notice certain peculiarities of 

 stratigraphical distribution. Thus the restriction of the genus Bronteus to the 

 Stinchar Limestone Group in the Ordovician is remarkable, and it is the earliest 

 known occurrence of the genus in the British Isles. A species from the Opper 

 Bala of Kildare, l>. andersoni from the Saugh Hill, Camregan, and IVnkill 

 Groups, and another from the Wenlock Limestone of Dudley, are the only other 

 representatives of the genus till we reach the Devonian, where its maximum 

 development takes place. The allied genus Bronteopsis appears to be confined to 

 the Balclatchie Group, and is only known in the Girvan area. 



The large number and variety of species belonging to the genus lllaenus is a 

 feature on every stratigraphical horizon ; but, unlike Bronteus and Bronteopsis, the 

 species are represented by abundant individuals in nearly every case. 



Remopletirides occupies a much more prominent position than is usual, and in the 

 Balclatchie Group members of the species 11. barrandei and ti. salteri and of the 

 peculiar Teratorhynchus bicornis are very numerous. 



Certain genera, or even families, are, on the other hand, much less developed 

 than on corresponding horizons in England and "Wales. Thus the genus Asaphus 

 is only represented by two species, and these are confined to the two lowest 

 groups of beds — the Stinchar Limestone and Balclatchie Groups ; and the 

 Phacopidas are rare in genera, species, and individuals; the genus Dalmanites, in 

 fact, is totally wanting. 



The vertical distribution of the genera and species is illustrated in the following 

 table, the occurrence of each species being indicated by a cross X. This table 

 thus indicates the relative development of the different genera on the successive 

 stratigraphical horizons, and brings conspicuously forward some interesting facts 

 with regard to vertical range and variable representation. The Ardwell Group, 

 the Barren Flagstone Group, and the Drumyork Group are omitted from the 

 list of formations as they have so far yielded no trilobites. It must be remem- 

 bered that the dominant genera of trilobites, as expressed by the number of species 

 in each stratigraphical division, do not- necessarily correspond with the dominant 

 species of trilobites as indicated by the abundance of individuals. Statistics are 

 not available to determine the "frequency values" of the various species; ' and 

 although without special collecting with this object in view we cannot give a true 

 and connected account of the development, movements, and adjustments of the 

 trilobites in the Girvan area through the Ordovician and Silurian periods, yet 

 certain changes in the proportionate specific representation of the genera and in 



1 Williams, ' Bull. 210, U.S. Geol. Surv.' (1903), pp. 10—41. 



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