170 



PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



Ludlow. 



392 species, representing 137 genera, is, as nearly as can be ascer- 

 tained, the numerical value of the Ludlow fauna. No true Ludlow 

 rocks are certainly known to occur in North "Wales, excepting 

 perhaps in a small part of Montgomeryshire, south-east of Newtown. 

 The Ludlow formation is zoologically allied to the Wenlock through 

 129 species. The chief classes, or those most represented numeri- 

 cally, are the Actinozoa with 12 genera and 17 species, the Echino- 

 dermata with 12 genera and 21 species, the Crustacea with 29 genera 

 and 97 species, the Brachiopoda with 13 genera and 48 species, the 

 Lamellibranchiata with 17 genera and 71 species, the Gasteropoda 

 with 9 genera and 33 species, the Cephalopoda 6 genera and 37 species, 

 and lastly the Vertebrata, which first appear through the Pishes, 

 represented by 10 genera and 14 species, omitting the genus OncJius, 

 the remains of which may be " telsons " or tail-spines of Crus- 

 tacea. The comparatively small fauna of the Aymestry Lime- 

 stone (53 genera and 84 species) is, although local, a remarkable 

 one, and clearly shows the nature of the sea-bed and sedimentary 

 material in relation to the habits of life of the species represented. 

 Mostly it is a recurrent fauna from the Wenloek, 57 species being 

 common to the two, these 57 representing 38 genera, clearly showing 

 the rarity and paucity of species illustrating the genera, and the 

 lenticular or accidental nature of the Aymestry Limestone between 

 the Lower and Upper Ludlow beds. The classes that appear to be 

 most fully represented are the Ccelenterata (Actinozoa), the Crusta- 

 cea, Brachiopoda, Gasteropoda, and Cephalopoda. No fish-remains 

 are known, although the two Ludlow series contain them *. The 

 feeble representation of the Ludlow rocks in North Wales, as before 

 stated (only 19 species), by comparison with Westmoreland (127) 

 calls for critical examination into the physical rather than the palae- 

 ontological aspect of the question ; and this is still more prominently 

 seen through the still larger fauna in South Wales and Shropshire, 

 the former yielding 163 species, the latter 187 ; and further through 

 the intermediate areas termed Herefordshire (where 114 species are 

 known) and Worcestershire (121). Could we but see and examine 

 the continuity of the old Wenlock and Ludlow sea-bed and its 

 deposits between Denbighshire, Westmoreland, and the south of 

 Scotland, now under the St.-George's Channel (or denuded away), 

 we should doubtless find traces of the Lower Wenlock and Ludlow 

 rocks, uniting the two now disunited areas. We cannot doubt that 

 this continuity once existed, though probably the rocks of the two 

 areas were deposited under very different and local conditions. 

 Petrologically they essentially differ ; but palseontologically or speci- 

 fically in certain groups the agreement is close and decisive, omit- 

 ting the largely represented Annulose or Crustacean orders Meros- 



* Mr. Salter included the Aymestry Limestone with the Lower Ludlow, as 

 being a calcareous condition of it. Mr. Lightbody was always impressed with 

 the fact that the calcareous nature of the beds above the Aymestry Limestone 

 influenced the fobsils to a marked degree. 



