the Geological Structure of North Wales. 155 



paratively few species undescribed in the work of Mr. Murchison. 

 It is therefore neither Silurian nor Cambrian in the limited sense in 

 which the words were first used ; but it represents both systems, 

 inseparable, as they are in nature, from one another. 



The upper division of the Cambrian slate series is so obviously 

 Upper Silurian, that the author adds very few details in illustration 

 of those given in the previous parts of the paper. He however sub- 

 joins the following list of fossils, chiefly derived from the beds of the 

 third section from Gam Brys to Abergele through this division. 



1 . In the highest beds near the north end of the section, — Encri- 

 nites, Graptolites Ludensis, Orthoceras virgatum, Leptcena lata, L. 

 depressa, Terebratula lacunosa, Orthis orbicularis, fyc. 



2. South of Bettws Abergele, and rather lower in the series,— 

 Asaphus caudatus, Orthis orbicularis, Leptcena lata, Terebratula lacu- 

 nosa, Atrypa affinis, Cardiola interrupta, fye. 



3 . In the valley of Llanfair Talhaiarn and Bronhaulog mines, — 

 Encrinites and branching corals, Bellerophon trilobatus, Leptcena lata, 

 Spirifer ptychodes, Orthis orbicularis, Atrypa affinis, Turbo corallii, #c, 



4. Plas Madoc quarry near Llanrwst, under the great mass of the 

 Denbigh flagstone, and therefore low in the descending section. 



Corals: — Fenestella, #<?., Calymene Blumenbachii*, Asaphus sub- 

 caudatus, Bellerophon globatus •, Leptcena euglypha, L. lata, Atrypa affinis, 

 Spirifer ptychodes, Orthis orbicularis, 0. lunata, Terebratula navicula, 

 T. lacunosa, fyc. 



5. To these lists the author adds the following from Dinas Bran 

 near Llangollen, the upper part of the Denbigh flags. 



Calymene (?) (in small fragments), Lituiti (in fragments), Ortho- 

 ceras striatum, O. ibex, 0. Ludense, Bellerophon expansus, Terebratula 

 navicula and lacunosa (in great abundance), Orthis lunata, Nucula 

 ovalis, Cypricardia retusa, Cardiola interrupta (?), Turbo carinatus, &;c. 



The Denbigh flags contain impressions of Orthoceratites ; but 

 some which have been referred to that genus are considered by Mr. 

 Forbes as species of Criseis, a genus of Pteropoda. They occur in 

 the middle of the group associated with Graptolites Ludensis, and 

 sometimes with Leptcena lata, Cardiola, and other fossils. 



From these lists it appears that the fossils of the upper division of 

 the great Welsh series agree very closely with the fossils figured by 

 Mr. Murchison, from the Wenlock shale to the tilestone at the base of 

 the old red sandstone inclusive. But the mineral structure of the 

 rocks being almost entirely different from that of the upper system 

 of Siluria, the distribution of the species is also different ; so that 

 they are incapable of being arranged into distinct groups marking 

 the successive parts of an ascending or descending section, so as to 

 agree with the subdivisions of Mr. Murchison. The author confirms 

 this remark by stating, that he now possesses more than ninety 



* We see no reason for abrogating the very convenient rule which has 

 always prevailed since the establishment of the Linnaean terminology, of di- 

 stinguishing such specific names as are substantives by a capital letter. We 

 do not therefore adopt the mode of printing Calymene blumenbachii, or 

 Orthis vespertilio ; but shall continue to follow the universal practice of 

 naturalists in all countries. — Edit. 



