150 



M.K. HOWARTH 



typical features of the species. The more evolute P. aureum occurs in 

 the middle part of the subzone, and the more finely ribbed P. obsoleta 

 is represented by only one specimen in Bairstow's collection from 

 the lower part of the subzone. Polymorphites trivialis is common in 

 the lower and middle parts of the subzone, and there are rare 

 occurrences of Radstockiceras, Parinodiceras and Tragophylloceras. 



Jamesoni Subzone, beds 550-560.3 (except the top 0.08m). The 

 base of the Jamesoni Subzone is placed at the first occurrence of 

 Uptonia jamesoni in bed 550, and the index species then extends 

 through the full thickness of the subzone, to which it is confined. The 

 more finely ribbed U. lata occurs less commonly in the upper half of 

 the subzone, and Polymorphites bronni is also characteristic of the 

 upper half of the subzone. The only other ammonites in the subzone 

 are the highest occurring specimens of Platypleuroceras brevispina 

 in bed 550. Polymorphites polymorphus at the middle of the subzone, 

 and rare examples of Parinodiceras and Tragophylloceras. 



IBEX ZONE, Masseanum and Valdani Subzones, beds 560.3 (top 

 0.08 m)-577. These two subzones are delimited according to the 

 distribution of species of Tropidoceras and Acanthopleuroceras: 

 Tropidoceras occurs in both subzones and Acanthopleuroceras only 

 in the Valdani Subzone. Unfortunately ammonites are rare in this 

 interval in Robin Hood's Bay, and the few examples of these genera 

 are not well-preserved. None, however, have the definite bituberculate 

 ribs of Acanthopleuroceras, so they all have to be identified as 

 Tropidoceras, in which umbilical tubercles are much reduced or 

 absent. The lowest example (PI. 7. fig. 10) from the top 0.08 m of bed 

 560.3 and another (PI. 7, fig. 1 1 ) from near the bottom of bed 568 are 

 best determined as T futtereri (Spath), while several specimens from 

 the boundary of beds 567 and 568 have the much more massive ribs 

 at larger sizes and the more widely spaced ribs of T. masseanum 

 (d'Orbigny), var. rotundum (Futterer). The base of the Masseanum 

 Subzone (and of the Ibex Zone) is placed 0.08 m below the top of bed 

 560.3 to include this earliest T. futtereri, and bed 568 probably 

 belongs to the same subzone. 



In the absence of Acanthopleuroceras there is no good evidence 

 for the position of the base of the Valdani Subzone, so it is placed 

 provisionally at the bottom of bed 571 from the occurrence of 

 Liparoceras (L.) heptangulare. There are two more specimens of 

 that species in beds 575 and 577. According to Spath ( 1938: 59) L. 

 (L.) heptangulare might be confined to the Valdani Subzone (ie. 

 Spath's 'Centaurus Subzone' ), so its presence in beds 57 1-577 (7.66 

 m thick) suggests that they are probably of Valdani Subzone age. 



The only other ammonites in either subzone are two Liparoceras 

 (L.) cheltiense low in the Masseanum Subzone, one Tragophylloceras 

 loscombi high in the same subzone, and Lytoceras fimbriatum in the 

 upper part of the Masseanum Subzone and throughout the Valdani 

 Subzone. The latter species becomes more common in the Luridum 

 Subzone. 



Luridum Subzone, beds 578. 1-580. The presence of eight Aegoceras 

 (Beaniceras) luridum in beds 578.1, 578.5 and 580 is sufficient 

 evidence to refer beds 578 to 580 to the Luridum Subzone. Other 

 ammonites in this subzone are a single Liparoceras (L.) cf. 

 naptonense, two Liparoceras (L.) sp. indet. and 14 examples of 

 Lytoceras fimbriatum. 



DAVOEI ZONE, Maculatum Subzone, beds 58 1-590.7. The base 

 of this zone and subzone is placed at the bottom of bed 581 which 

 contains the lowest Aegoceras (A.) maculatum (PI. 7, fig. 12). Two 

 more, typical, examples occur in bed 582.3, then there are many 

 well-preserved specimens at higher levels, especially in beds 590.6 1 

 and 590.63. Other ammonites in the Maculatum Subzone are A. (A.) 

 maculatum vars atavum and leckenbyi. Liparoceras (L.) cf. 



naptonense, Androgynoceras heterogenes and Lytoceras sp. indet. 

 See remarks on the identification of Aegoceras maculatum (p. 141) 

 for discussion of the division of the Maculatum Subzone into smaller 

 units. 



Capricornus Subzone, beds 59 1-596. 1 . The base of the subzone is 

 placed at earliest occurrence of Aegoceras (A. ) lataecosta in bed 59 1 . 

 This is 1.83 m above the highest A. (A.)maculatum in bed 590.63, but 

 the intervening strata (beds 590.64-590.7) did not yield any ammo- 

 nites and are retained in the Maculatum Subzone. The only other 

 ammonites in the subzone are A. (A.) artigyrus, which has more 

 massive whorls and coarser ribbing than lataecosta, and a number of 

 poorly preserved Aegoceras (A.) sp. indet. 



Figulinum Subzone, beds 596.2-600.5. This subzone is based on 

 the range of the subgenus Oistoceras. The index species, Aegoceras 

 (Oistoceras) figulinum, occurs in beds 600.2 and 600.4 near the top 

 of the subzone, but the base of the subzone is placed at the lowest 

 appearance of A. (O.) sinuosiforme in bed 596.2. This and A. (O.) 

 angulation in bed 600.2 have more widely spaced ribs thanfigulinum, 

 especially on the inner whorls. The only other ammonite in the 

 subzone is a single Liparoceras (L.) divaricosta in bed 596.3 (PI. 8, 



fig. 1). 



The top of the subzone is limited by the base of the Stokesi 

 Subzone ( Margaritatus Zone, Upper Pliensbachian ), which is placed 

 at the first appearance of Amaltheus stokesi in bed 600.6. There are 

 other examples of A. stokesi in bed 600.8 and at higher levels in the 

 Stokesi Subzone. Aegoceras (Oistoceras) figulinum and Amaltheus 

 stokesi are confined to their respective subzones in Robin Hood's 

 Bay and their ranges do not overlap. 



Acknowledgements. I wish to thank the late Leslie Bairstow for en- 

 trusting me with his unfinished manuscripts in the hope that 1 would be able 

 to complete them in a form suitable for publication. Mrs L.M. Spencer kindly 

 helped me to retrieve those manuscripts when he left his London home in 

 1985. Thanks are also due to Mr. Peter Jones of King's College, Cambridge, 

 for information about Bairstow during his time at that college, to Rosalind 

 Moad for allowing me to examine the original copy of Bairstow's 1930 

 Fellowship Dissertation, which is held in the Archive Centre at King's 

 College, and to Professor D.T. Donovan for reading the manuscript and 

 suggesting several significant improvements. 



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