lib YORKSHIRE TYPE AMMONITES— II Jan. 



91. PORPOCERAS VERTICOSUM, nov. 

 (Plate XCI) 



Original Description 

 Ammonites vorticellus, Yds. a, Simpson, 1855, p. 61 (Y.T.A. 90). 

 46, 24, 33, 



Proportions, {£ £ 33, g| 



Substeno-, subpachy-gyral, subextremilatumbilicate. 



Conch, serpenticone ; periphery, i ; ornament, 5. 



Whorls subrotnnd, with inflated sides ; periphery just arched ; 

 umbilicus very open, undulate. Ribs somewhat distant, somewhat 

 coarse, showing intermittent tuberculation — on last whorl, closer, smaller, 

 sharper ; fibulate pattern occasional. Tubercles when present often give 

 rise to 3 ribs on periphery, where there is somewhat of a zigzag pattern, 

 the tubercles being mpre or less alternate, not truly opposite. 



Distinction 



From A. vorticellus (Xo. 90), stouter at the same diameter, with 

 stronger and coarser ornament ; the same features distinguish P. vortex 

 (Xo. 29) from this species. 



P. vortex, P. verticosum, P. vorticellum form a catagenetic series, 

 the strength of ornament, which is decreasing in the species, decreases 

 in the series ; so does the size of the species. The whorl decreases in 

 breadth in each species, as well as in the series as a whole. The umbilicus, 

 though it increases in size in each species, does not attain the same degree 

 of size in the smaller species, though it begins its increase earlier. The 

 following proportional figures show the changes in individuals and series. 



Remarks 

 The present species, which Simpson regarded as a variety of his 

 Am. vorticellus, is the Am. subarmatus of d'Orbigny and Joly. 

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

 Geological position, presumably Alum Shale. 



Result 

 Porpoceras verticosum, nom. nov. Whitbian, [fibulatum zone, 

 near Whitby]. 





