gob YORKSHIRE TYPE AMMONITES— II Nov. 



90. AMMONITES VORTICELLUS, Simpson 

 (Plate XC) 



Original Description 

 " 81. A[mmonites] vorticellus. [M. Simpson, 1855, P- 61.] 



["I. Without a dorsal keel or furrow." p. 35. 

 " b. Armed with spines or distinct tubercles." p. 58.] 



" Volutions 6 or 7, exposed, outer whorl \ the diameter ; radii 

 annular, obtuse, equal to the intervening concave spaces, alternate 

 radii form a tubercle near the outer margin of the whorl, then split in 

 two, and pass directly across the back ; aperture subquadrate ; diameter 

 I J inch. 



"This is a much more slender and depressed species than the last 

 [A. vortex, Y.T.A. 29.]. 



" Another, rather thicker, with the radii on the inner whorls more 

 distant, each having a small tubercle on the outer margin of the whorls, 

 may, for the present, be placed here as Var. a." 



Simpson, 1884, 93, typographic variations only. 



Remarks 



Proportions, 40, 21, 27, 56 ; substeno-, subpachygyral ; sub- 

 extremilatumbilicate. 



Stages, conch, serpen ticone ; periphery, 1 ; ornament, 5. 



Whorls quadrate, sides flattened, periphery barely arched. Ribs 

 somewhat distant, not strong. Ornament regular tuberculate in inner 

 whorls, intermittent tuberculate later ; fibulate pattern only feebly 

 indicated. Primary rib when tubercled bifurcate over periphery ; 

 otherwise it is entire. 



Genus, Porpoceras (I, v) ; family, Dactyloidse. 



Geological position, presumably Alum Shale and fibulatum zone. 



The Var. a referred to above is a different species — see next article. 



Result 

 Porpoceras vorticellum, Simpson sp. 1855, Whitbian, [fibulatum 

 zone, near Whitby]. 



