part 4] JUEASSIC CHEOXOLOGT : LIAS. 318 



Turning now to the biological argument — the biological facts 

 are: 1, Microderocerata which are developments of M. hirclii; 

 2, Arietites Avhich are developments of A. turneri : the geo- 

 logical facts are a special matrix for these developments and the 

 position of M. hirchi-A. turneri on the Dorset Coast: the geo- 

 graphical facts are the presence of the special fauna (and a distinc- 

 tive matrix) only at isolated places in England, the absence of 

 the fauna from well-known, much-searched localities, and its ver}^ 

 limited range upon the Continent. 



On these various grounds it may be suggested that the full 

 Mercian-Lymian sequence, instead of being 



3. Arietites hrooki, 



2. Arietites turneri, 



1. Microderoceras bircJii, 



is rather 



^ 5. Arietites brooki, 



4. Arietites turgescens {-\-A. aff. turgescens), 



3. Arietites turneri, 



2. Microderoceras injlatum-\-M. septigerum+M. depressum, 

 1, Microderoceras hircM. 



This involves the supposition of non-sequences between hirclii 

 and turneri and again between turneri and hrooki, on the Dorset 

 Coast and over other wide areas ; but it may be unnecessary to 

 suppose a non-sequence at Barrow for, I think, turneri forms 

 were found there. Then the absence of Arietites hrooki and 

 Microderoceras hirchi from Barrow might be explained by 

 supposing that the excavation began below the hrooki bed, and did 

 not go so deep as hirchi ; but other explanations are possible : the 

 area just north of the Mendips was in constant movement. 



Another suggestion ma}^ be made : that the sequence is 



5. Arietites hroolci. 



4. Arietites turgescens. 



3. Microderoceras inflatxim. 



2. Arietites turneri. 



1. Microderoceras hirchi. 



This would require only one non -sequence at Lyme Eegis, and 

 would meet the case if the true A. turneri has not been found at 

 Barrow. This sequence shows faunal repetition such as that found 

 in the Raasayan and in the Hwiccian-Wessexian. Nos. 3 and 4 

 might be called the Barrow Beds. 



In the first case the division between Mercian and Lymian 

 would be drawn between 2 and 3. In the second case, however, it 

 would seem desirable not to draw it between 2 and 1 as pro})osed 

 earlier in this paper (pp. 271, 274), but to draw it below 1, to 

 prevent divorce of the Microderocerates. 



In the case of Bredon there is evidence that earlier faunas were 

 uncovered than at Barrow; and there is reason to suppose that 

 they were found in the peculiar chalky Lias, also that there was 



