116 FISHES OF THE SILURIAN ROCKS, 



modern Silurian deposit, then (No. 1 of the accompanying 

 diagram), is the Upper Ludlow rock ; and it is in the su- 

 perior strata of this division that the bone-bed discovered in 

 1838 occurs; while the exceedingly minute vertebrate re- 

 mains described by Professor Phillips in 1842 occur in its 

 base. The division next in the descending order is the Ay- 

 mestry Limestone (No. 2) ; the next (No. 3) the Lower Lud- 

 low rock ; then (No. 4) the Wenlock or Dudley Limestone 

 occurs ; and then, last and oldest deposit of the Upper Silu- 

 rian formation, the Wenlock Shale (No. 5). It is in the 

 fourth or Wenlock Limestone division that the defensive 

 spine described in the Edinburgh Review for 1845 as the 

 oldest vertebrate organism known at the time was found ;* 

 while the vertebrate organism found by Professor Phillips 

 belongs to the fifth or base deposit of the Upper Silurian. 

 Further, the American spines of Onondago and Oriskauy, 

 described in 1846, occurred in rocks deemed contemporary 

 with those of the Wenlock division. We next cross the line 

 which separates the base of the Upper from the top of the 

 Lower Silurian deposits, and find a great arenaceous forma- 

 tion (No. 6), known as the Caradoc Sandstones ; while the 

 Llandeilo Flags (No. 7), the formation upon which the sand- 

 stones rest, compose, according to the sections of Sir Rode- 

 rick, published in 1839, the lowest deposit of the Lower Si- 

 lurian rocks. And it is in the upper part of this lowest 

 member of the system that the ichthyic defences announced 

 in 1847 by Professor Sedgwick occur. Vertebrate remains 

 have now been detected in the same relative position in the 

 seventh and most ancient member of the system that they were 

 found to occupy in its first and most modern member ten 

 years ago. But this is not all. Beneath the Lower Silurian 

 division there occur vast fossiliferous deposits, to which the 



* "The shales alternating with the Wenlock Limestone." {Edin 

 iurgh Eeview.) 



