part 4] 



JUEASSIC CHROXOLOaX : LIAS. 



271 



(2) Mercian. 



The strata of this date are Avell developed in Grioucestershire 

 and the Midlands — mainl}^ the ancient kingdom of Mercia. The 

 Mercian corresponds to the catagenetic phases of the Arietidie 

 (post-tuberculate Asteroceras and incoiling phases of Arietites). 

 In middle Mercian is a phase of degenerate Arietids which have 

 lost keel and furrows — usually called degenerate Asterocerates. 

 The fauna is very prominent in Yorkshire, and is found in Ireland 

 and in Wilrtemberg. It is not, however, known from other British 

 localities, with the exception of Pershore (Worcestershire) ; but the 

 special Pershore degenerate is a species not known from tlie other 

 localities. On the principle of dissimilar faunas it is reasonable to 

 expect one, if not two, horizons in middle Mercian somewhat later 

 than ohtusum. - 



In early Mercian the Barrow- Grurney fauna has its own pecu- 

 liarities, possibly indicating another horizon. But this cannot be 

 discussed without a consideration of the Ammonites ; so it must 

 be relegated to the Palseontological Appendix, see p. 311. 



In regard to Arietites furneri, Mr. Tutcher is of opinion that 

 it precedes hirchi} and this would seem to be the position assigned 

 to it by Tate & Blake. It is true that the name A. turneri has 

 been ver}^ much misused, and that in a ^Ye-hirchi position there 

 are, as I have reason to think, forms allied to Arniocerns Avith 

 somewhat of an Arietites aspect which possibly require separation 

 as a new genus from both. But an explanation on these lines 

 does not satisf}'' me, where Mr. Tutcher is eoncei-ned ; fauna! 

 repetition on the lines of Echioceras of the Kaasayan may be 

 more likely. It is evident that a prolonged stratigraphical and 

 palaeontological investigation is required to solve the problem. 



The following table gives the evidence of faunal distj-ibution 

 and sequence in the Mercian. 



Table VII. — Meeciax : Scotland asd Exglaxh. 

 Dates. Strata. 



Heraerse. 





Raas 



AT. 



YOEKSKIRE. 



I Cheltenham: 



DlSTRICT.- 



Bristol 



Dj STRICT. - 



Dorset 



COAST.- 



7. denotattis .. 









I 





XCi!elteiiham. 







6. stellare 













XCheltenliain. 



Radstoek. 



88 



5. planicosta .. 









9 







Radstock. 



85-87 



4. sagittarmm 









10 





X Pershore. 







3. ohtusum 









10 





X Bredou. 



Radstock. 



83,84 



2. brooJci 





E. 













7«-«9, 



1. turneri 



1 'Geol. 



Wl 



R. 



litby ' 



op. 



Jam cif. p. 



67. 



X Bredon. 



- Asi 



X Barrow 76, 77 

 Gurney. 



n previous Tables. 



See Appendix I, p. 280. 



