G. H. Morton — Range of Carboniferous Species. 133 



In North Wales the occurrence and succession of the species in 

 the subdivisions vary in diffei'ent areas, and the larger the area 

 examined the more difficult it becomes to find species peculiar to 

 certain horizons. In order to obtain a definite result I have 

 compiled three separate lists of the species obtained in that number 

 of distinct areas. There are the Llangollen, the Flintshire, and 

 the Vale of Clwyd Lists. Each of the lists shows the relative 

 scarcity and abundance, and the range of the species in the sub- 

 divisions ; and although future search will doubtless add to the 

 rare and occasional species, the number and range of the common 

 and very common must be very nearly correct. Neither the rare 

 nor the occasional species are of much use in defining distinct 

 horizons in consequence of their rarity, and it is only the common 

 and very common species that can be expected to indicate a definite 

 horizon or zone. In North Wales a great difficulty arises from the 

 occurrence of all the common and very common species in the 

 Upper Grey Limestone, with the exception of Productas comoides, 

 and although all pass downwards, they become scarce in consequence 

 of the general paucity of fossils in the inferior subdivisions. 



In the Llangollen List there are 69 rare, 28 occasional, 16 common, 

 and 27 very common species. Deducting Foraminifera, which are 

 not in the other lists, there are 36 species that are common and very 

 common, and they all occur in the Upper Grey Limestone, with the 

 exception of Posidonomya Gihsoni from higher strata, and Productns 

 comoides in the Lower Brown Limestone, all the other species in the 

 list being rare and occasional forms. 



In the Flintshire List there are 92 rare, 35 occasional, 30 common, 

 and 11 very common species ; and of the 41 common and very 

 common, 37 species occur in the Upper Grey Limestone, 3 of the 

 remaining species, Posidonomya Becheri, AvicaJopecten granosus, and 

 A. papyraceiis, occurring in the Upper Black Limestone, and. Fro- 

 ductus comoides in the Lower Brown Limestone. 



In the Vale of Clwyd List, which includes the Great Orme's 

 Head, there are 16 rare, 22 occasional, 12 common, and 10 very 

 common species; and of the 22 common and very coinmon, 21 

 species occur in the Upper Grey Limestone, the exceptional species 

 being Productus comoides. None of the 21 species are peculiar to 

 the subdivision, for they all occur in the underlying Middle White 

 Limestone. The number in the list is less than in the others, on 

 account of the Upper Grey Limestone having been considerably 

 denuded in the Vale of Clwyd. 



Nearly the whole of the common and very common fossils occur 

 in each of the three lists, for there are few that are not found in all 

 the areas. 



Of the numerous common and very common species found in the 

 Carboniferous Limestone of North Wales, it is impossible to find 

 any that are restricted to horizons of less importance than the 

 subdivisions into which the formation is naturally divided. An 

 examination of the first appearance and continuity of the species 

 seems to indicate that they were introduced from some pre-existing 



