26b YORKSHIRE TYPE AMMONITES Mar. 



26. AMMONITES FIGULINUS, Simpson 

 (Plates XXVI a. b.) 



Original Description 

 " 45. A[mmonites] figulinus. fM. Simpson, 1855, pp. 47, 48.] 



[" I. Without a dorsal keel or furrow. 

 " a. No spines." p. 35.] 



"This also [like A. omissus'] has sharp radii, strongly bent towards 

 the aperture on the back ; but the whorls are much stronger, and the 

 radii separated by wider and deeper concave spaces [than in A . omissus] ; 

 the aperture between the radii would be circular, but if taken in a line 

 through the radii, it would be many sided, for the outline from the inner 

 edge of the whorl is first convex, then, for a short space, it becomes 

 concave, so as to [p. 48] form two angles on the side of the whorl ; it then 

 undulates across the back, where the radii seem as if rubbed up with 

 the finger in a plastic state." 



Simpson, 1884, p. 78, the same. 



Remarks 



Stages, conch, serpenticone ; periphery, i ; ornament 5**. 



The angles that Simpson mentions are tubercles : there are an inner 

 and outer row of small tubercles from about 15 mm. diameter — the inner 

 row perhaps not so soon. 



The " radii strongly bent towards the aperture on the back " 

 [v-shaped radii of periphery] form a distinctive generic character in this 

 and several other species. As there is no generic name available, that of 

 Oistoceras is proposed (Gen. p. iv). Family Liparoceratidae. 



The geological position is not recorded : it is presumably Lower 

 Lias, a — d. 



Result 



Oistoceras figulinum, Simpson sp. 1855, Charmouthian, [striatum 

 zone], near Whitby. 



Note 



Plate XXVIb illustrates a Dorset specimen to shew the coarse- 

 ribbed swollen whorls of an individual more mature than Simpson's type. 

 There are, on the periphery of Simpson's specimen, indications of the 

 commencement of such a whorl. This is the curvicornis or bituberculate 

 swollen stage. The peripheral view of this specimen shews the character- 

 istic V-shaped costje. From near Lyme Regis, evidently from the Green 

 Ammonite Bed (striatum zone). 



