LOWER LIAS OF ROBIN HOOD'S BAY 



143 



Range. Two specimens in bed 583.2, Maculatum Subzone. 



Remarks. This record is based on two specimens (C. 38873-74) 

 from bed 583.2 that were determined by Spath ( 1938: 1 33) as belong- 

 ing to his var. atavum; two other specimens in the same bed 

 (C. 34887 1 -72) are typical of the normal variety of maculatum. C. 38873- 

 74 are both only 34 mm diameter, possibly nearly adult, with reduced 

 ribbing on the nearly flat venter (PI. 7, fig. 8). The whorl breadth is not 

 as large as in Spath's (1938: 127, pi. 20, fig. 3) type of his variety, but 

 otherwise they are closely similar. Spath ( 1 938: 1 28) himself remarked 

 that an alternative place for such specimens might be in the genus 

 Beaniceras, and it is possible that bed 583.2 might be the horizon from 

 which the holotype of Aegoceras (Beaniceras) luridum was obtained 

 (SM J3274, figured Dean et al, 1961: pi. 69, fig. 6). That holotype is 

 somewhat larger (46 mm diameter), more complete and better pre- 

 served than Bairstow's specimens, and no others that are as 

 well-preserved have been found in beds 578-583. Spath's determina- 

 tion as var. atavum for these Bairstow collection ammonites is retained 

 here. 



Aegoceras (A.) maculatum (Young & Bird), var. leckenbyi 

 Spath, 1938 



1938 



1938 



Androgynoceras maculatum (Young & Bird), var. leckenbyi; 

 Spath: 126, pi. 13, fig. 2 (C.3741, from bed 590.1). 

 Androgynoceras maculatum (Young & Bird), var. arcigerens 

 (Phillips); Spath: 126, pi. 20, fig. 5 (from Dorset). 



Range. A single specimen in bed 590. 1 , Maculatum Subzone. 



Remarks. This variety is kept distinct from the normal type of A. 

 maculatum only because it develops massive whorls with 'Liparoceras- 

 type' of ornamentation at sizes of more than 100 mm diameter. 

 Bairstow's specimen is 120 mm diameter, and a Dorset specimen with 

 similar whorls at 120-150 mm diameter was figured by Spath ( 1938: 

 pi. 20, fig. 5) as A. maculatum var. arcigerens (Phillips). 



Aegoceras (A.) lataecosta (J. de C. Sowerby, 1827) 



1 827 Ammonites lataecosta J. de C. Sowerby: 1 06, pi. 556, fig. 2. 

 1880/82 Aegoceras lataecosta (J. de C. Sowerby); Wright: 365 



(1882), pi. 32, fig. 1 (1880) (holotype, BM 43916, from 



Drift, locality unknown). 

 1938 Androgynoceras lataecosta (J. de C. Sowerby); Spath: 135, 



pi. 19, figs 4 (holotype, BM 43916), 6 (BM C.38562, from 



Staithes, Yorkshire). 



Range. Single specimens in beds 59 1 and 594, Capricomus Subzone. 

 Aegoceras (A.) artigyrus (Brown, 1837) 



1837 

 1843 

 1855 

 1884 

 1889 

 1938 



1973 



1973 



Ammonites artigyrus Brown: 26, pi. 19, fig. 5. 

 Ammonites defossus Simpson: 15 

 Ammonites defossus Simpson: 48 

 Ammonites defossus Simpson: 78 

 Ammonites artigyrus Brown: 20, pi. 19, fig. 5. 

 Androgynoceras artigyrus (Brown); Spath: 158, pi. 14, fig. 

 5, pi. 18, fig. 1, pi. 23, figs 3 (holotype, Manchester Museum 

 LL.230, possibly from bed 593), 12. 14. 

 Aegoceras maculatum (Young & Bird); Donovan & Forsey: 

 14, pi. 4, fig. 1 (SM Bl 1945, paralectotype of Ammonites 

 defossum Simpson, 1843). 



Aegoceras (Oistoceras) cf. figulinum (Simpson); Donovan 

 & Forsey: 14, pi. 4, fig. 2 (SM B 1 1 946, lectotype of Ammo- 

 nites defossum Simpson, 1843, designated by Donovan & 

 Forsey). 



1985 Androgynoceras (Aegoceras) capricomus (Schlotheim), 

 morphotype A. artigyrus; Phelps: 352, pi. 2, fig. 6. 



Range. Found by Bairstow only in bed 593, Capricomus Subzone, 

 4 specimens, but Phelps' figured specimen is from bed 595.2. 



Remarks. Donovan & Forsey's (1973: 14) designation of the Robin 

 Hood's Bay specimen SM B 1 1946 as lectotype of Ammonites defossus 

 Simpson, 1843, has consequences for the position of both the genus 

 Defossiceras Buckman, 1913, and Simpson's species defossus. That 

 lectotype and the paralectotype (SM B 1 1945), also figured for the first 

 time by Donovan & Forsey, are closely similar to each other, and both 

 have robust, quickly expanding whorls and moderately fine ribs on the 

 inner whorls. The robust whorls are not like the slender whorls of 

 Oistoceras at similar sizes, and the ribs are more closely spaced on the 

 inner whorls than in Aegoceras (A. ) maculatum. The ribs are projected 

 slightly forwards on the venter of both lectotype and paralectotype of 

 defossus, but a varying amount of projection of the ribs on the venter 

 occurs in Capricomus Subzone species of Aegoceras (A.), though it is 

 never as pronounced as in the later subgenus Oistoceras. The holotype 

 of A. (A. ) artigyrus figured by Spath (1938) has similarly robust whorls, 

 the same rib-density on the inner whorls and similar slight forward 

 projection of the ribs on the venter. This is considered here to be the best 

 place for defossus, and places the genus Defossiceras as a junior 

 synonym of Aegoceras (A.) (ie. not a synonym of the subgenus 

 Oistoceras), and the species defossus as ajunior synonym of Aegoceras 

 (A.) artigyrus. 



Subgenus BEANICERAS Buckman, 1913 



Aegoceras (Beaniceras) luridum (Simpson, 1855) 



1855 Ammonites luridus Simpson: 46. 



1913 Beaniceras luridum (Simpson); Buckman: pi. 73 (holotype, 

 SM J3274). 



1961 Beaniceras luridum (Simpson); Dean et al.: pi. 69, fig. 6 

 (holotype, SM J3274, possibly from bed 583.2) (see de- 

 scription of Maculatum Subzone, p. 150 below). 



Range. Beds 578. 1-580, Luridum Subzone; 8 specimens; possibly 

 also from bed 583.2, Maculatum Subzone. 



Remarks. Although the eight specimens in beds 578 and 580 are 

 crushed, they are much larger (up to 37 mm diameter) and more com- 

 pressed than A. (B.) centaurus (d'Orbigny ), and so can be confidently 

 referred to A. (B.) luridum. 



Subgenus OISTOCERAS Buckman, 1911 



Aegoceras (Oistoceras) sinuosiforme Spath, 1938 



PI. 7, fig. 14 



1938 Oistoceras sinuosiforme Spath: 167, pi. 1 8, fig. 6, pi. 19, fig. 

 7 (holotype, BM C.38564), pi. 26, figs 6, 7, 9 (all Spath's 

 figured specimens are from Lincolnshire). 



Range. Beds 596.2-598. 1 , Figulinum Subzone; 93 specimens. 



REMARKS. A.(0.) sinuosiforme has more widely spaced ribs and 

 much less well-developed chevrons on the venter than A. (O. ) figulinum. 



Aegoceras (Oistoceras) angulatum (Quenstedt, 1856) 



1856 Ammonites maculatus angulatus Quenstedt: 121, pi. 14, fig. 

 12. 



1885 Ammonites maculatus angulatus Quenstedt; Quenstedt: 270, 

 pi. 34, fig. 11. 



