2ob YORKSHIRE TYPE AMMONITES Mar. 



20. AMMONITES LENTICULARIS, Young & Bird 

 (Plate XX) 



Original Description 

 [Young & Bird, 1828, p. 269.] 



" The last shell of this family [thin discoid ammonites, with sharp 

 keels] which we shall name is more lenticular than any that we have seen. 

 The exterior part of the whirl runs to a thin edge, plain or very faintly 

 crenated ; the sides are smooth, or marked with very faint undulating 

 Unes ; the central part is an umbilicus, with upright sides, the inner 

 whirls being scarcely visible ; and the aperture forms a triangle, of 

 which the outer angle is extremely acute, owing to the thinness of the 

 edge. This rare species, found in the lias bands, may fitly be termed 

 A. lenticularis." 



Remarks 



Stages, conch, oxycone ; periphery, 2c ; ornament, ic. 



A few small portions of complicated suture-hnes are to be seen : 

 they show narrow-stemmed lobes, the laterals tridactyloid. There is 

 general agreement with the suture-hne of Amaltheus (Quart. Journ. 

 Geol. Soc, 1889, xlv, 4, 5) ; shape and indications of a crenulate keel 

 agree with that ; but the very slight projection of the radial Hne on 

 the periphery and the comparative smoothness of test are against 

 Amaltheus. If it be an Amaltheus, it is a very catagenetic form. 



Its geological position is Middle Lias e (Simpson, 1884, 115). 



Result 

 Amaltheus lenticularis. Young & Bird sp., 1828, Domerian, 

 spinatum zone, near Whitby. 



