i6b YORKSHIRE TYPE AMMONITES Mar. 



i6. AMMONITES AMBIGUUS, Simpson 

 (Plate XVI) 



Original Description 

 " 3. A[mmonites] ambiguus. [M. Simpson, 1843, p. 8.] 



[" I. Without a dorsal keel or furrow. 

 " a. No spines." p. 7.] 



" Much depressed ; volutions 4, inner ones Vo concealed, outer 

 whorl more than half the diameter ; radii twice bent, numerous, faint ; 

 aperture ovate ; diameter 2 J inches. 



This differs from the next [A. erratus] in the faintness of the radii, 

 and in the regular convexity of the sides of the whorls ; it is also thinner 

 near the back, and the upper edge of the inner margin of the whorl 

 projects over the inner one. The markings on the shell, which is very 

 thin, differ very little from those on the cast. The stria bend in the same 

 direction as the radii. I beheve it to be from the lower lias. Another 

 specimen I obtained from the lower beds of Marlstone at Robin Hood's 



Bay-" 



Additional Details 



Simpson, 1855, p. 36, — instead of second par. has, " The markings 

 on the shell, which is very thin, differ very little from those on the cast. 

 L. L. — R. H. Bay." Simpson, 1884, p. 64, describes the shell as 

 " A. loscombei, Sow.," and says, " L. L., w, R. H. Bay, Huntcliff," but 

 enters A. loscombei in t in p. xx. 



Remarks 



Stages, conch, oxycone ; periphery, 2(c ?) ; ornament, 2C. 



There is no true carina, but a sharpened periphery crossed by small 

 ribs, having therefore a crenulate appearance. There are signs of 

 phylliform septation. 



Genus, Rhacoceras, Aga.ssiz, in Hyatt, 1867, 86. 



Geological position, w = capricornum zone, t = valdani zone : 

 A. loscombei-Wke forms have this range, but the higher zone {w) seems 

 most probable in this case. 



Result 



Rhacoceras ambiguum, Simpson sp., 1843, Charmouthian \capri- 

 cornum-zone ?], Robin Hood's Bay, near Whitby. 



