76b YORKSHIRE TYPE AMMONITES— II August 



76. AMMONITES DEFOSSUS, Bean-Simpson 

 (Plate LXXVI) 



Original Description 

 " 21. A[mmonites] defossus, Bean's MSS. [M. Simpson, 1843, p. 15]. 



["I. Without a dorsal keel or furrow. 

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



" Depressed ; volutions 5 or 6, exposed ; radii prominent, sharp, 

 straight, suddenly bend towards the aperture in passing over the back, 

 where they are much flatted ; intervening spaces flattish ; striated ; 

 aperture quadrate ; diameter if inch. 



" This has long been confounded with the last [A. maculatus, 

 Young & Bird], which it much resembles. As it occurs so frequently 

 in this form, there can be little doubt that it is a distinct species. It may 

 readily be distinguished from the last by the flatted back, which in some 

 places is even concave. The whorls are less numerous than in 

 A. arcigerens. It is from the Marlstone series." 



Additional Details 



Simpson, 1855, 48, " next " for " last " in first sentence, 2nd par. = 

 A. maculatus ; " last " in 3rd sentence = A. figulinus. Instead of last 

 sentence, " M.L." Simpson, 1884, p. 78, " next " in first sentence, 

 2nd par. = A. capricornus ; ends 2nd par. with " L.L., d, R.H. Bay," 

 instead of " M.L." 



Remarks 



Proportions, 42, 29, 31, 47 ; subplaty-, subpachygyral, perlatum- 

 bilicate. 



Stages, conch, serpenticone ; periphery, 3, incipient ; ornament 5*. 



This is a comparatively oligogyral species. The first whorl is costate 

 and stout, with a small umbilicus, almost a costate cadicone ; later the 

 whorl is rounded and the umbilicus enlarges, later the whorl becomes 

 somewhat quadrate. 



The ribs on the flattish sides have a slight curvature : they begin 

 at whorl-edge versiradiate, then just beyond rounded umbilical edge 

 have a slight rursiradiate direction ; then, near the tuberculate genicular, 

 gradually curve forward to sweep over the flattened squared periphery 

 V-wise : the V looks as a ridge of clay would appear when pushed forwards 

 by the finger; it is medianly flattened and broadened, while there are 

 signs of a breaking up into subsidiary v ribs. A small carina, not of 

 continuous strength, joins the apex of each V. 



Genus Dejossiceras, nov. (II, vi) ; family Amaltheidae. 



Geological position, Simpson's L.L. d is presumably intended for 

 L.L. d, that is striatum zone. The specimen is of a brownish colour, 

 however, not dark black nor maculate like some other species from that 

 zone, so it may be somewhat later. B. luridum (No. 73) has a similar 

 colour. 



Result 

 Defossiceras defossum, Bean-Simpson sp. Charmouthian [capri- 

 cornum or] striatum zone, near Whitby. 



