^32 



FOSSILS OF UPPER LUDLOW. 



[Ch. XXYII. 



beneatli the lowest strata of the " Old Red." Some of the fish are of the 

 shark family, and their defences are referred to the genus OncJius (fig*. 

 569). There are also numerous minute shagreen scales (fig. 5*70), whicli 



Fi?. 569. 



Onchus ienuistriatu,s, Agass. 

 Bone-bed. Upper Silurian ; Ludlow. 



Fig. 570. 



Shagreen scales of a placoid fi 



( Thelodm). 



Bone-bed. Upper Ludlow. 



Plectrudus mirahilis, Agass. 

 Bone-bed. Upper Ludlow. 



may possibly belong to the same placoid fish. The jaw and teeth of 

 another predaceous genus (fig. sVl) have also Fio-. 5T1. 



been detected. As usual in bone-beds, the 

 teeth and bones are, for the most part, frag- 

 mentary and rolled. Many statements have 

 been published of fish remains obtained from 

 older members of the silurian series ; but Mr. Salter has shown all 

 these to be spui'ious.'* Pi'ofessor Phillips has, however, discovered fish- 

 bones at the bottom of the " Upper Ludlow," at its junction with the 

 Aymestry Rock ;f and lower than this no one seems as yet to have suc- 

 ceeded in tracing them downwards, whether in Europe or jSTorth America, 

 for M. Barrande's most ancient ichthyolites (bony fragments, 8 inches 

 long) occur in the Upper Siknian of Bohemia ; and those of the American 

 Geologists are from the Oriskany Sandstone, a formation which is still 

 considered as debatable ground between the Devonian and Silurian sys- 

 tems (see p. 426, above). 



In England it is true, as in the United States and Canada, globular, 

 cylindrical, or flattened masses have been detected, composed principally 

 of phosphate of lime, in the Lowest Silurian rocks, and they have been 

 suspected to be coprolitic. Messrs. Logan and Hunt have recently shown 

 that shells of the genera Lingula and Orhicula^ which occur abundantly 

 in the same formations, are also made up of phosphate and carbonate of 

 lime, mixed in the like proportions ; and it has been suggested that the 

 decomposition of such shells might give rise to the nodules alluded to, 

 which may owe their form to concretionary action.^ Even if the zoologist 

 should think it more likely that the phosphatic matter was rejected in 

 foecal lumps, by creatures feeding on Lingulae and Orbiculse, we cannot 

 decide that such feeders were of the vertebrate class, rather than. Cepha- 

 lopods. Crustaceans, or some other of the Invertebrata. In regard to the 

 doctrine of the supposed non-existence of fish in the Silurian seas before 

 the time of the Ludlow bone-bed, I shall consider that question fully in 

 the concluding pages of this chapter, p. 453, et seq. 



* Geo! Quart. Journ. vol. vii. p. 263. 



f Memoirs Geol. Surv. vol. ii. 



\ Logan and Hunt ; Silliman's Journ. No. 50, 2d series, March, 1854. 



