120 MIDDLE ALBIAN STRATIGRAPHY 



Price, the formal proposal of Kilian and the employment of the name by Collignon 

 (1963 ; 2) given precision by P., & J. -P. Destombes, has at last taken root. H. (H.) 

 benettianus is not a satisfactory subzonal index for in the strict interpretation it may 

 be restricted to only a comparatively narrow horizon within the Subzone that it is 

 supposed to represent. They have proposed a type locality for this Subzone, the 

 Tuilerie Clerc at La Vendue-Mignot, Aube (p. 95), where the Lyelliceras fauna is con- 

 sidered to be in the pure state without risk of contamination (in terms of collecting 

 fossils that is). In fact the section does not show the relationship between either the 

 eodentatus Subzone below or the spathi Subzone above, and there are other objections 

 already mentioned. 



The benettianus Subzone was defined without precision by Spath (1923a ; 4, 1923b ; 

 73, 1926b ; 422), and was used by him for a range of sediments some of which are 

 truly lyelli in age but others have subsequently proved to be of spathi age. It would 

 be far more satisfactory if the proposal of P., & J. -P. Destombes were adopted 

 modified to a single index of Lyelliceras lyelli. 



In France, the finest section now available in sediments of this Subzone is certainly 

 that of Courcelles pres Clerey, Aube (p. 91), although that at Les Cotes Noires is 

 probably the most completely displayed in terms of sediments. The best develop- 

 ment seen in England was exposed at the Horton Clay pit near Small Dole, Sussex 

 (p. 35). A comparison of these two sections brings out the nature of the ammonite 

 fauna and the differences to be found between them. Lyelliceras camatteanum and 

 its related species of the eodentatus Subzone is connected with Lyelliceras lyelli by a 

 series of morphological transitions. Intermediate forms such as Lyelliceras pseud- 

 olyelli (Parona & Bonarelli non Spath), Lyelliceras huberianum (Pictet) and L. 

 hirsutum (Parona & Bonarelli) form such transitional species. These show the 

 transition from the extreme en-echelon arrangement of the ventro-lateral rib termin- 

 ations and non-tuberculate siphonal fine characteristic of Tegoceras, to the single ribs 

 commencing at the umbilical margin and sweeping without break straight across the 

 periphery and bearing mid-lateral, ventro-lateral and siphonal clavi typical of 

 Lyelliceras lyelli. These transitional forms occur right at the base of the lyelli Sub- 

 zone both at Courcelles and to a limited extent at Small Dole, and on balance it seems 

 that the time span in which they existed was of comparatively short duration. Un- 

 condensed basal lyelli Subzone sediments are also apparently well developed in the 

 Cote d'Or (e.g. Ciry 1927 : Ciry, Rat, Malapris & Nicolas 1965). 



It is apparent that ecological conditions have a marked effect upon the ammonite 

 fauna. In deposits of the lyelli Subzone containing a benthonic fauna, the hetero- 

 morph ammonites Protanisoceras (P.) barrense and P. (P.) alter notuberculatum are 

 common (e.g. Small Dole, and Courcelles) and are as equally characteristic of this 

 time span as is Lyelliceras lyelli. Where no benthos is present, or it is very reduced 

 in numbers, the heteromorph ammonites are absent or very rare (e.g. the Isle of 

 Wight). Lyelliceras lyelli is also affected by ecological conditions in that in beds 

 containing abundant Hoplites (H.), Lyelliceras is not common. Casey has already 

 noted the tendency to mutual exclusiveness between the earlier members of these 

 two families (1957 ; 43-44), although this could in part be due to the hoplitids being 

 able to withstand more adverse conditions in the seas of that time. However, the 



