152 THE HABITAT OF THE EURYPTERIDA 



area of over a thousand square miles and yet it never exceeds and 

 seldom reaches 1 foot in thickness. 



Following the Upper Ludlow in England comes a series of forma- 

 tions of no very great thickness which has been subdivided into the 

 Tilestones, Downton Castle sandstones, and Ledbury shales. Mur- 

 chison applied the name "Tilestones" to the whole of the flaggy 

 upper parts of the Ludlow, and since many of the beds are red he in- 

 cluded them in the Old Red Sandstone. They were believed to 

 mark a transition period between the Upper Siluric and the Lower 

 Old Red, but to be more like the latter with which they were therefore 

 classed. The Downton sandstones are a group of red, yellow and gray 

 micaceous rocks from 80 to 100 feet thick, occurring in the neighbor- 

 hood of Downton Castle, Herefordshire, and also supposed to mark a 

 transition period. They are undoubtedly indicative of the regressive 

 movement of the sea, which began in Lower Ludlow time in Scotland 

 but which was not strongly felt in England till the end of the Upper 

 Ludlow. Then in the Downtonian and other "passage beds" were 

 washed into the deposits, terrestrial and lycopodious, vegetal remains, 

 together with eurypterids, Ceratiocaris and vast numbers of Bey- 

 richia kloedeni, together with Lingula cornea and Platyschisma 

 helicites. 



In Scotland all of the beds above the Upper Ludlow are called, by 

 the Geological Survey of Great Britain, the "Downtonian." This 

 series is to be looked upon as " stratigraphical equivalents of the Tile- 

 stones, Downton sandstones and Ledbury shales which, in Hereford- 

 shire, overlie the Upper Ludlow Rocks and have been classified as 

 forming the highest subdivision of the Upper Silurian rocks" (215, 

 568). It is evident that such a usage of Downtonian will lead to 

 endless confusion, for not a little misunderstanding has already arisen 

 because some authors have placed the English Downton beds in the 

 Lower Old Red, and others have used Downton and Passage Beds 

 almost synonymously. If an attempt is made to use Downtonian 

 in a comprehensive way, coordinate in importance with the terms Wen- 

 lock and Ludlow, then difficulties will arise and much circumlocution 

 will be necessary to explain whether the Downton of England or the 

 Downtonian of Scotland is meant and in correlation difficulties will 

 come about because so many different deposits are known by the 

 same name. And especially does it seem inadvisable to adopt a name 

 which in England is used for a subdivision of the Ludlow, and make 

 it in Scotland of the same rank as Ludlow. Therefore, the author 



