550 C. Lapivorth — On Scottish MonograptidcB. 



On an inspection of the foregoing table it is at once apparent that 

 the Scottish Monograptid^ noticed in this paper fall into three well- 

 marked specific groups. The first group embraces those of the 

 Birkhill Shales ; the second those of Gala and Girvan ; and the third 

 those occurring in the Upper Silurians of Kiccarton and the Pentland 

 Hills. The first and second of these groups overlap each other to 

 a large extent — at least half the species of the Gala rocks being 

 survivors from the Birkhill fauna. Remembering the fact that the 

 Birkhill and Gala formations pass almost insensibly the one into the 

 other by a gradual lithological and palseontological transition, this 

 large community of species appears extremely natural. Between 

 the second and third groups, however, there is a decided palseonto- 

 logical break — only two of the Gala forms occurring in the Eiccarton 

 Beds. This also is easily accounted for, as the intervening Hawick 

 beds, which would naturally yield the forms common to the two 

 groups, has not as yet afforded any examples of Graptolites. 



Group A. — In Scotland this group falls into two divisions — the 

 earlier of which is mainly distinguished by the absence of those 

 Birkhill forms which survive into the Gala Group ; and by the ex- 

 clusive presence of Dimorpliograptus elongatus, Monogr. Sandersoni, 

 Monogr. fimhriatus, Monogr. triangulatus, and the typical form of 

 Bastrites peregrinus. 



Outside of South Scotland this minor assemblage occurs in its 

 typical form only in the Coal-pit Bay beds of County Down. 



The later division contains a large admixture of Gala forms. 

 As yet its only exact foreign representative occurs in the lowest zone 

 of the Upper Graptolite Schist of Sweden. 



The forms of Group A. considered as a whole, are met with in 

 abundance in the Coniston Mudstone of the North of England, 

 which ought in all probability to be looked uj>on as the exact repre- 

 sentative of the Birkhill Shale of South Scotland ; if, indeed, it be 

 not a portion of the same wide-spreading Graptolitic deposit. As 

 this Coniston Mudstone reposes upon the representative of the Bala 

 Limestone of North Wales, it is probably of Lower Llandovery age. 

 This aids us in fixing the age of the Birkhill Shales, which must 

 thus form the first member of the Scottish Middle Silurian. The 

 same assemblage of species re-appear in the Kiesel Schiefer of 

 Saxony and Thuringia, i.e. in the zone of Black Shale upon which 

 reposes the great mass of the Middle Silurian of that district. 



Group B. — This embraces the forms found in the great Greywacke 

 Series of the Gala Group, and is made up of several survivors of 

 the Birkhill fauna, some peculiar species, and two or three Upper 

 Silurian forms. The peculiar species include Cyrtograptus Grayi, 

 Jlonogr. Galaensis, Monogr. crispus, Ilonogr. Barrandei, Monogr. 

 exigiAus. Of fossils passing upwards into the Upper Silurian only 

 two are yet known : Monograptus Hisingeri and Monogr. priodon. 



This assemblage of forms is unknown outside of South Scotland, 

 but if we suppose the Hawick rocks to yield the Upper Silurian 

 forms which are found in association with Gala species at Tieveshilly 

 (Ireland), etc., in beds which appear to come into the place of the 



