9§ 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



genera and 36 species in the Lower, and 16 genera and 41 species in 

 the Upper Lingula-flags, only 8 genera and 8 species unite the two 

 groups : they are 5 Trilobita and 3 Brachiopoda — namely Anopolenus 

 SaZteri, Hicks, Microdiscus punctatus, Salt., Agnostus princeps, Salt., 

 Paradoxides Hicksii, Salt., P. aurora, Salt., Lingulella Davisii, L. 

 lepis, and Orthis sagittalis. Only 4 classes are represented in the 

 Upper Lingula-flags, the Annelida, Crustacea, Bryozoa, and Brachio- 

 poda. The remaining 10 have no representative whatever. 16 

 genera and 41 species occur in the Upper Lingula-beds ; 8 genera and 

 30 species are Crustacea, and 5 genera and 8 species Brachiopoda ; 

 there are 2 Annelida, and 1 BryozooD, Dictyonema. 9 genera and 

 10 species pass to the Lower Tremadoc. 



Lower Tremadoc. — Table V. The Lower Tremadoc fauna nume- 

 rically consists of 28 genera and 58 species; and the fauna is 

 compact and characteristic. Only 8 genera and 9 species appear 

 from the Upper Lingula-flags below : 4 of them are Crustacea ; and 



5 are Brachiopoda, and, with Dictyonema, complete the incomiug 

 species. The one remarkable feature in the group is the presence 

 and first appearance of 5 genera and 12 species of Lamellibranchiata. 

 Dr. Hicks discovered them in the Upper Tremadoc beds on Ramsey 

 Island. They mark an epoch in the history of the class. Davidia 

 and Glyptarca are new genera, Modiolopsis, Palcearca, and Cteno- 

 donta receive Mr. Hicks's 8 other species. Until this class is 

 found in lower beds, these 12 species identify the Lower Tre- 

 madoc of llamsey Island as an important horizon in time and space. 

 Only 4 of the 13 genera and 4 of the 24 species of Crustacea pass 

 to the Upper Tremadoc ; they are Agnostus princeps, Ampyx prai- 

 nuntias, Olenus alatus, and DUcelocephalus furca ; and 4 genera and 



6 of the 12 species of Brachiopoda also connect the Upper with 

 the Lower Tremadoc • they comprise the following — Lingula pe- 

 talon, Lingulella lepis, Obolella Belti, 0. plicata, Orthis Carausii, 

 and 0. lenticularis. These and Theca ovata, T. bijugoscc, and T. eus- 

 pidata, in all 9 genera and 13 species, constitute the transgressing 

 fauna. 



Upper Tremadoc — Table VI. Nine whole classes are wanting 

 in this uppermost division of the Lower Cambrian rocks. Only 20 

 genera and 33 species compose the fauna of the Upper Tremadoc. 

 We have seen that 9 genera and 13 species pass from the Lower Tre- 

 madoc, thus leaving only 11 genera and 20 species as truly belong- 

 ing to the Upper Tremadoc. More than 50 per cent, of the species 

 pass to the Arenig, or 11 genera and 16 species. These few species 

 little affect the question of the palasontological break that takes 

 place here ; for 42 new genera and 133 new species make their ap- 

 pearance in the succeeding Arenig, through some physical changes 

 accompanying the zoological, which we have not yet been able to 

 satisfactorily discover. Unconformity between this group and the 

 Arenig is not known in the typical areas of South and North Wales ; 

 but at no time in the history of the lowest Palaeozoic rocks has appa- 

 rent extinction on the one hand, and migration from some unknown 

 area in the other, taken place so markedly. We have yet to learn or 



