92b YORKSHIRE TYPE AMMONITES— II Jan. 



92. AMMONITES OBSOLETUS, Simpson 

 (Plate XCII) 



Original Description 

 " 37. Afmmonites] obsoletus. [M. Simpson, 1843, p. 23.] 



["I. Without a keel or furrow." p. 7. 

 " b. Armed with spines or distinct tubercles." p. 22.] 



" There is also in the Whitby Museum another [besides A. andraei] 

 which has broader, flatter whorls ; the radii are very regular, formed 

 merely by a slight groove, each has a small knob near the outer margin 

 of the whorls, and then becomes nearly obsolete on the back ; diameter 

 4! inches. 



Additional Details 



Simpson, 1855, pp. 59, 60 — " 77. A. obsoletus, Simp. — Much 

 depressed ; volu-[p. 6o]tions 5, exposed, outer whorl less than J the 

 diameter, sides flatted, inner margin suddenly rounded ; radii numerous, 

 straight, nearly obsolete, terminate in a row of small tubercles on the 

 outer margin ; diameter 4J inches. 



' The radii are very regular, formed merely by a slight groove. 

 I have seen several, probably of this species, quite flatted. — L.L. ; 

 R. H. Bay." 



Simpson, 1884, pp. 91, 92 — p. 92 adds to first par. " L.L. 14, 

 R. H. Bay [Robin Hood's Bay] ", and omits locality from 2nd ; but 

 adds after ' flatted ' — " They are figured by T. & B. [Tate & Blake], 

 pi. vii., f. 1." 



Remarks 



Proportions, 92, 26, 16, 48 ; subplaty-, lepto-gyral, perlatum- 

 bilicate. There is nearly another half-whorl ; but this cannot be satis- 

 factorily measured, as it is displaced and damaged. 



Stages, conch, serpenticone ; periphery, 1 ; ornament, 5. 



The whorls are thin, laterally flattened except for very slight arching ; 

 the greatest thickness about the middle. The inner margin, sharp, 

 flattened. 



Ribs are small, feebly elevated, rounded, with the slightest sign 

 of lateral curvature ; each ends in a small spine, which is just at edge 

 of periphery and is not covered. Radii appear to continue over venter 

 as fine lines with slight forward curve. Ventral view cannot be given 

 because of hard matrix. 



Genus, Uptonia, S. Buckman (I. viii) ; family, Polymorphidae. 



Geological position, presumably jamesoni zone, with other species 

 of the genus. 



Simpson was mistaken in citing Blake's figure ; and Blake was wrong 

 in his identification. His Mgoceras obsoletum is one of the Am. densinodus 

 group, family Deroceratidae, and is from a much lower horizon. The 

 position given by Simpson is about correct for Blake's species ; but is 

 much too low for an Uptonia. 



