552 LUDLOW FORMATION. [Ch. XXVII. 



helicites, Bellerophon trilobatus, Chonetes lata, &c, with numerous 

 defences of fishes. These beds are well seen at Kington in Hereford- 

 shire, and at Downton Castle near Ludlow, where they are quarried 

 for building. 



Bone-bed. — The bone-bed of the Upper Ludlow deserves especial 

 notice as affording the most ancient example of fossil fish occurring in 

 any considerable quantity. It. usually consists of one or two thin 

 brown layers full of bony fragments near the junction of the Old Red 

 Sandstone and the Ludlow rocks, and was first observed by Sir R. 

 Murchison near the town of Ludlow, where it is three or four inches 

 thick. It has since been traced to a distance of 45 miles from that 

 point into Gloucestershire and other counties, and is commonly not 

 more than an inch thick, but varies to nearly a foot. At May-Hill 

 two bone-beds are observable, with 14 feet of intervening strata full 

 of Upper Ludlow fossils.* At that point immediately above the 

 upper fish-bed numerous small globular bodies have been found, which 

 were determined by Dr. Hooker to be the sporangia of a cryptogamic 

 land-plant, probably lycopodiaceous. These beds occur just beneath 

 the lowest strata of the " Old Red," forming the uppermost part of the 

 Downton sandstone. 



Most of the fish have been referred by Agassiz to his placoid order, 

 some of them to the genus Onchus, to which the spine (fig. 619) and 

 the minute scales (fig. 620) are supposed to belong. It has been sug- 



Fig. 619. Fig. 620. 



Onchus tenuistriatus, Agass. Shagreen scales of a placoid fisli 



Bone-bed. Upper Silurian ; Ludlow. ( Thelodus). 



Bone-bed Upper Ludlow. 



gested, however, that Onchus may be one of those Acanthodian fish, 



referred by Agassiz to his Ganoid order, which are so characteristic 



of the base of the Old Red Sandstone in 



Fig. 62i. Forfarshire, although the species of the 



Old Red are all different from those of the 



Silurian beds now under consideration.! 



is, Agass. The J aw and teetn of anotner predaceous 



Bone-bed Upper Ludlow. genus (fig. 621) have also been detected, 



together with some specimens of Pteraspis 



Ludensis. As usual in bone-beds, the teeth and bones are, for the 



most part, fragmentary and rolled. 



b. Gray Sandstone and Mudstone, &c. — The next subdivision of 

 the Upper Ludlow consists of gray calcareous sandstone, or very com- 



* Murchison's Siluria, pp. 13'7-23'7. 



f Powrie, Geol. Quart. Journ., vol. xx. p. 438. 



