r2b YORKSHIRE TYPE AMMONITES Mar. 



12. AMMONITES LEVISONI, Simpson 

 (Plate XII) 



Original Description 



" 105. A[mmonites] Levisoni. [M. Simpson, 1843, p. 54.] 

 [" III. Keel between two furrows," p. 48. 

 " b. Furrows distinct." p. 50. J 

 "This ammonite much resembles the last {A. walcottii, Sow., 

 A. hildensis, Young & Bird], of which it may possibly be only a variety ; 

 the sides of the whorls are more rounded, the inner margin more depressed, 

 and the groove on the side is wanting ; the radii also are finer and more 

 numerous, and commence at or near the inner margin of the whorl : 

 the shell is thin, smooth, and shining ; the keel is sharp, and it has 

 altogether a much more elegant appearance than the last : it occurs 

 in the marlstone and ironstone series. I have named this ammonite 

 in remembrance of my mineralogical friend, Mr. Levison, whose friend- 

 ship for all lovers of science is warm and disinterested." 



A dditional Details 



Simpson, 1855, jjp. 99, 100. — " 184. levisoni. Simp. — Volutions 5, 

 inner ones but [p. 100] httle concealed, outer whorl \ the diameter, inner 

 edge rounded ; radii sigmoidal, obtuse, equal to the intervening concave 

 spaces ; keel entire, furrow on either side distinct ; aperture ovate ; 

 diameter ij inches." 



Simpson, 1884, p. 140, the same. 



Remarks 



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



The carina appears to be non-septate ; the ribs run more or less 

 distinctly across sloping inner margin from contact hne ; they are some 

 times almost connate in inner whorls. 



The genus is Hildoceras, Hyatt, 1867 — family Hildoceratidae. The 

 geological position is, in spite of what Simpson says, probably the Hard 

 Shale. Blake (1876, 307) assigns it to his serpentinuni zone ; but 

 his identification of the species is not to be trusted, nor are others. 

 Simpson's phrase, " radii commence at or near inner margin," was mis- 

 leading : it is only partially correct, unless, as is hkely, he meant in this 

 case the line of contact. 



Result 



Hildoceras levisoni, Simpson sp., 1843, Whitbian [falciferutn 

 zone ?], near Whitby. 



