LOWER LIAS OF ROBIN HOOD'S BAY 



149 



found in the subzone, along with widely scattered specimens of 

 Tragophylloceras numismale. 



Polymorphus Subzone, beds 538-544.5. The first appearance of 

 Polymorphites marks the base of this subzone. The earliest species is 

 P. caprarius which occurs in the bottom half of the subzone, fol- 

 lowed by P. trivialis in the upper half. The latter species extends into 

 the Brevispina Subzone, and other species occur in the Jamesoni 

 Subzone. The only other ammonites in this subzone in Robin Hood's 

 Bay are the last examples of Tragophylloceras numismale, a single 



Hype rde roc eras sp. indet. low in the subzone and the oxynoticeratid 

 genus Radstockiceras: the small species R. sphenonotum is confined 

 to the Polymorphus Subzone, and another much larger fragment of a 

 Radstockiceras was found in bed 544.4 near the top of the subzone. 



Brevispina Subzone, beds 544.6-549. The base is placed at the first 

 appearance of Platypleuroceras brevispina, which is common 

 throughout the subzone and extends up to its highest occurrence in 

 the basal bed of the Jamesoni Subzone. Many of those in beds 544 

 and 546 are large, crushed and fragmentary, though they show the 



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Fig. 25 Distribution of ammonites in the Ibex and Davoei Zones, Lower Pliensbachian. and in the base of the Margaritatus Zone, Upper Pliensbachian, of 

 Robin Hood's Bay. 



