IN THE ANGLO-PARIS BASIN 127 



all the Gault below Bed 13 of Destombes was of Middle Albian age, but it must now be 

 recognised that Bed 12 (v) below is in fact of basal cristatum Subzone age. 



The fauna of the cristatum Subzone on balance, bearing in mind that a late daviesi 

 Subzone element is present in Bed VIII (i), has its most important faunal link with 

 the beds above rather than those beneath. The very characteristic lower Upper 

 Albian bivalve form /. sulcatus develops in the basal part of the cristatum Subzone and 

 ranges up to the top of the orbignyi Subzone. Towards the top of the Middle Albian 

 daviesi Subzone, extreme forms of Anahoplites of the daviesi group occur which are 

 close to, but still generically distinct from Epihoplites (including Metaclavites). At 

 the same point in time species of Enhoplites and Dimorphoplites modify towards those 

 of the cristatum Subzone. The hoplitinids are almost completely dominant, the onfy 

 Mojsisovicsiinid being Dipoloceras cornutum which is very rare. In the cristatum 

 Subzone above we see the introduction of an important non-hoplitinid element in the 

 ammonite fauna. In fact many of the subfamilies last well-represented in the Anglo- 

 Paris basin in lyelli Subzone times appear once again in consort, and as before are 

 subordinate to the hoplitinids. 



Euhopiites is well represented but the species are essentially different to those of 

 the Middle Albian below, and are, moreover, more closely linked to those of the 

 orbignyi Subzone. This genus survives until the auritus Subzone and it is interesting 

 to find that a largely unfigured varicosum Subzone fauna in the Leighton Buzzard 

 area contains forms which are morphologically closer to those species of Bed V-VII 

 at Folkestone than to those of Bed VIII. Dimorphoplites in the strict sense does not 

 survive the cristatum Subzone, its nitch being filled by Epihoplites (including Meta- 

 clavites Casey) . It is quite possible that the ecologic factors which caused Inoceramus 

 concentricus to develop the strengthened shell from of i". sulcatus, also brought about 

 changes in the morphology of the endemic hoplitinids during the clearly unsettled 

 physical conditions in the cristatum Subzone sea. 



The non-hoplitinid element in the ammonite fauna is subordinate but highly 

 characteristic of this Subzone and foreshadows the major development of the branco- 

 ceratinid and mortoniceratinid ammonites which occurs at the base of the orbignyi 

 Subzone in the sense used here. Hysteroceras is represented by H. pseudocornutum, 

 H. capricornu, and H. simplicicosta, forms which are transitional from the earlier 

 Eubrancoceras. An early mutation of H. orbignyi does in fact occur in the cristatum 

 Subzone, but the only species of Mortoniceras known is M. rigidum although the 

 generic position given it by Spath is uncertain. Dipoloceras itself, the characteristic 

 genus of this time span, probably gave rise to Mortoniceras but the picture is not 

 clear at this time. The Lyelliceratidae is represented by Neophlycticeras which also 

 ranges into the orbignyi Subzone but it is rare. Beudanticeras represented in the 

 cristatum Subzone by the type-species B. beudanti and forms such as B. subparandieri, 

 also continued on into the orbignyi and higher Upper Albian Subzones. 



The cristatum Subzone must also include the time span represented within the 

 sediments of the basal part of Bed IX at Folkestone up to the level at which Hyster- 

 oceras and Mortoniceras characteristic of the orbignyi Subzone suddenly became 

 dominant. This just excludes the horizon of Euhopiites inornatus which is of wide- 

 spread occurrence in England and forms a good marker for the base of the orbignyi 



