54b YORKSHIRE TYPE AMMONITES May 



54. AMMONITES TENELLUS, Simpson 

 (Plate LIV) 



Original Description 

 " 178. A[mmonites] tenellus. [M. Simpson, 1855, p. 97.] 



I" III. Keel between two furrows, p. 90. 

 " b. Furrows distinct." p. 93.] 



" Much depressed ; volutions 5, inner ones J concealed, outer whorl 

 less than ^ the diameter ; radii separated by widish spaces, straight, 

 turn suddenly towards the aperture near the back ; striae, or lines of 

 growth, numerous, fine, beautifully sharp and distinct upon the keel, 

 which they finely crenate ; keel prominent, round, relieved from the sides 

 of the whorl by a concavity ; aperture triangular or ovate ; diameter 

 2f inches. 



" This is a more elegant species than the last [A. impendens, Y. & 

 B.] ; the radii are less prominent, and the groove on the side of the 

 keel much flatter.— L.L. ; R. H. Bay." 



Simpson, 1884, 137, the same. 



Remarks 



Stages, conch, becoming oxycone ; periphery, 4c ; ornament, 4c -3c. 



The periphery is concavifastigate. The carina bears fine hair-like 

 lines (strise, Simpson). The greatest thickness of the whorl at aperture 

 is II mm., at edge of periphery, 7 mm. 



Genus, Arietites, Waagen, (Gen. vi) ; family, Arietidae. The 

 geological position is presumably about L.L. 16. 



Result 

 Arietites tenellus, Simpson sp. 1855, Sinemurian, [stellare zone], 

 Robin Hood's Bay, near Whitby. 



IS 



