Ob YORKSHIRE TYPE AMMONITES ' "Mar. 



1910 



g. AMMONITES ALTERNATUS, Simpson 

 (Plate IX) 



Original Description 



" 82. A[MM0NITES] ALTERNATUS. [M. SiMPSON, 184J, pp. 4J, 44.] 



[" II. With a keel on the back. 

 " a. Outer whorl broad." p. 31.] 



" Depressed ; volutions 4 or 5, inner ones J concealed, [p. 44] sides 

 flatted, a row of obtuse depressions near the inner margin, which is sloping, 

 outer whorl | the diameter ; radii numerous, twice bent ; keel sharp, 

 entire ; aperture acutely triangular ; diameter 3 inches. 



This ammonite may be easily distinguished from A. elegans, 

 A. exaratus, and others of a similar form, by the depressions upon the 

 inner margin of the whorls, as well as by the less breadth of the outer 

 whorl, and by its being thicker towards the back ; also, where the shell 

 remains, there may be observed very slender radii in the furrows, on 

 which account it has received the above name. I have seen only one 

 specimen, it was obtained by T. Crosby, from the jet rock at Hawsker 

 Bottoms." 



Additional Details 

 Simpson, 1855, pp. 86, 87, instead of last sentence has " Jet-rock ; 

 U.L." (87) ; 1884, PP- 123, 124, adds to first par. " U.L. 7 " (124). 



Remarks 



Stages, conch, between serpenticone and oxycone ; periphery, 3c ; 

 ornament, mainly 3c. 



There is a septicarina ; in the side view the partition-band ends 

 where the keel is seen to begin : the rest of the keel has been broken off. 



The rib-curve and other characters distinguish the species from 

 Harpoceras, and make a new generic name, Harpoceratoides, necessary 

 (Gen. p. 7/). 



Result 



Harpoceratoides alternatus, Simpson sp. 1843, Whitbian, exara- 

 tum-zone, Hawsker Bottom, near Whitby. 



