nib YORKSHIRE TYPE AMMONITES— II Apr. 



in. AMMONITES OVATUS, Young & Bird 

 (Plates CXI a,b,c) 



Original Description 

 [Young & Bird, 1822, pp. 251, 327.] 



" [P. 251]. No. 4, PL xiii, belonging to the same strata [as 

 a. mulgravius], differs from No. 7 [a. elegans], chiefly in having the 

 internal angles rounded, giving the aperture an oval form. Hence we 

 may call it a. ovatus. 



" [P. 327]. Plate xiii. Fig. 4. Ammonites ovatus. Ditto [Alum Shale]." 



Additional Details 



Young & Bird, 1828, p. 265. " No. 4, PL xiii, from the hard bands 

 on the Hawsker shore, resembles A. striatulus, being marked with faint 

 sigmoidal ribs, and numerous sigmoidal striae, parallel to them. It is 

 also keeled, and has an oval aperture ; but its inner whirls are much 

 more concealed, and its aperture is much larger, being more than one-third 

 of the diameter. Where the shell is entire, the ribs are scarcely discernible, 

 but only the striae, which in the central part are curiously bent. This 

 specimen, which is not common, we have named A. ovatus. It is some- 

 times 9 or 10 inches broad. 



" [P- 359]- Plate xiii. . . . Fig. 4. A. ovatus. Ditto. [Lias bands] " 



Remarks 



The 1828 example is not the same as that used for illustration in 1822. 

 It also differs from it in many characters — it is more catagenetic, the 

 subcostate stage ends about a whorl earlier passing into a stage of distant 

 indistinct, undulate parvicostae on cast, but on test similar costae, on and 

 between which are strong growth lines (striae), ultimately only striae : 

 the radial line is more rostrate and in the striate stage becomes much 

 curved medianly : the inner margin is more flatly sloped and ultimately 

 becomes indistinct : the umbilicus is more contracted at the same diameter 

 (c. 100 mm.), but ultimately becomes proportionately larger. 



Geological position : The horizon of the 1828 example is the so-called 

 ovatus band ; for Simpson describes the 1828 example (1884, p. 112). 

 Possibly the 1822 form occurs with it ; but, as this was uncertain, the 

 designation pseudovatum was employed for this horizon (Y.T.A. 1910, xvi), 

 intended as the name for the 1828 species. It was listed as Pseudolioceras 

 pseudovatum, S. Buckman (Geol. Whitby, Ed. 2, Mem. Geol. Surv. 1915, 

 p. 93) in default of a distinctive generic name. 



Genus OVATICERAS, nov. (p. xi), the 1822 form the genotype. 



Result 

 Ovaticeras ovatum, Young & Bird sp., 1822, Whitbian [pseudovatum 

 zone], near Whitby, and Ovaticeras pseudovatum, S. Buckman sp., 

 1910 (Am. ovatus, Young & Bird, 1828, non 1822), Whitbian, pseudovatum 

 zone, near Whitby. 





