394 J. BARRELL INFLUENCE OE CLIMATES ON VERTEBRATES 



Ludlow and lies immediately below the beds of passage into the lowest of 

 the Old Red Sandstone formations — the Downtonian — uppermost Silu- 

 rian in age. The Ludlow is the uppermost division of the British Si- 

 lurian which holds marine fossils and the Downtonian sandstones repre- 

 sent the appearance of continental deposition in latest Silurian time. It 

 is in a layer of the Upper Ludlow from a few inches to a foot in thickness 

 and rich in carbonate of lime, phosphate of lime, iron, and bitumen that 

 most of the fossils are crowded and which constitutes what is called the 

 Ludlow bone bed. 6 The fauna, as listed by Phillips, 7 contains in notable 

 abundance one species of pelecypod, two of gasteropods, three of brachio- 

 pods, two species of shark spines belonging to the genus Onchus, four 

 species of Ostracoderms, and plant remains. For the whole of the Upper 

 Ludlow, Phillips gives ten genera and fourteen species of fishes, all 

 ostracoderms except for the two species of Onchus. 



The Downtonian rocks above the Ludlow are dominantly red and yel- 

 low, fossil-barren, false-bedded sandstone, representing the first outbuild- 

 ing of the subaerial Old Eed Sandstone deposits across this region. Rare 

 fossiliferous bands of a different sedimentary nature carry a brackish- 

 water fauna and represent such incursions of the shallow sea as occur 

 from time to time over the outer parts of all delta regions. This fauna 

 contains fishes which Traquair has determined to be sharklike ostraco- 

 derms associated with more typical ostracoderms; but no true acanthodian 

 sharks have been found. 8 



The sharklike genus of ostracoderms, Thelodus, has been traced upward 

 from the Ludlow rocks into the higher horizon of the Lower Old Red 

 Sandstone of the Caledonian basin at Turin Hill, where marine associa- 

 tions are no longer in evidence. Here are found, however, a number of 

 species of true acanthodian sharks, mostly of the genus Climatius. 9 It 

 is striking that this first well known shark fauna should be found in rocks 

 which are clearly non-marine. Their environment at that time appears 

 to have been somewhat more restricted than was that of the far lower 

 ostracoderms. Throughout the time of deposition of the Lower Old Red 

 Sandstone of the Caledonian basin, so far as the fossil record indicates, 

 the primitive sharks reign supreme, the ganoids and dipnoans not yet 

 appearing. From their sudden invasion, strong in numbers in the fol- 

 lowing Orcadian fauna, it is to be inferred that these subclasses were in 



8 Murchison : Siluria, 1859, pp. 148-153. 



7 John Phillips : Manual of geology. Edited by Etheridge and Seeley. Part ii, 1885, 

 pp. 127-138; especially p. 129. 



8 Trans. Royal Soc. Edinburgh, vol. xxxix, part iii, 1899, pp. 827-864. 



9 A. S. Woodward : Catalogue of fossil fishes, part ii, 1891. 



G. Hickling : The Old Red Sandstone of Forfarshire. Geol. Mag., Decade 5, vol. v, 

 1908, pp. 398-401. 



