14b 



YORKSHIRE TYPE AMMONITES 



Mar. 



14. AMMONITES RUDIS, Simpson 

 (Plate XIV) 



Original Description 

 " 83. A[mmonites] rudis. [M. Simpson, 1843, p. 44.] 



[" II. With a keel on the back." p. 31. 

 . " b. Outer whorl narrower." p. 44.] 



" Depressed ; volutions 4 or 5, inner ones ^ concealed ; radii 

 strong, straight, then turn towards the aperture, equal to the concave 

 spaces between ; aperture roundish ; diameter, 2J inches." 



" This is an extremely rugged shell ; the outer whorls are only 

 slightly indented by the succeeding ones. Near the inner margin of the 

 whorls are coarse irregular swellings, from whence proceed two radii, 

 which are occasionally separated by other radii, but with the greatest 

 irregularity. I have seen only one specimen, the shell of which is thick, 

 the keel is broken off, and the siphuncle is well displayed." 



Additional Details 

 Simpson, 1855, 87, the same ; 1884, 124, adds " U. L." 



Remarks 



Stages, conch, serpenticone ; periphery, 4c ; ornament, 5. 



The degree of tuberculation, the irregularity and coarseness of 

 costation, place this species between Am. robustus, Denckmann, and 

 Am. malagma, Dumortier (I, A, B, y — S. Buckman, Mon., p. x) ; so it 

 falls into the genus Denckmannia, S. Buckman (Id. xvii). The sHghtly 

 flexed character of ribbing noticed for this division of Denckmannia 

 is a feature of a later stage of growth than the present specimen shows. 



Result 



Denckmannia rudis, Simpson sp., 1843, Whitbian [variabilis-zone], 

 near Whitby. 



