29b YORKSHIRE TYPE AMMONITES Mar. 



29. AMMONITES VORTEX, Simpson 

 (Plates XXIX A.B.) 



Original Description 

 " 80. A[nimonites] vortex. [M. Simpson, 1855, i^y. 60, 61.] 



[" I. Without a dorsal keel or furrow." p. 35. 

 b. Armed with spines or distinct tubercles." p. 58.] 



Volutions 6 or 7, exposed, outer whorl \ the diameter ; radii numerous, 

 annular, split in two near the back, where there is a row of tubercles, 

 and pass directly over without bending towards the aj^erture ; aperture 

 subquadrate, transverse ; diameter 3 inches. 



" This resembles some varieties of A. subarmatus, having two 

 [p. 61] radii connected with each tubercle, in the button and loop style ; 

 the tubercles, however, are more distant, there being on the outer whorl 

 two radii interposed between those connected with the tubercles. The 

 inner edge of the whorls is also more rounded, forming a deep groove 

 at the suture, and the umbilicus is not so deep, the outer whorl having 

 nearly the same size throughout. The inner side of the whorls is but little 

 indented by the succeeding ones. The radii, which on the inner whorls 

 are strong and rounded, and equal to the intervening concave spaces, 

 become depressed and nearly obsolete towards the aperture. Where 

 the shell has been removed, there is displayed a very strong siphuncle, 

 without any apparent coastrictions. The ramifications of the septa are 

 numerous and pointed, differing very much from those observed in 

 A. subarmatus. — L.L. ; R. H. Bay." 



Additional Details 



Simpson, 1884, pp. 92, 93 ; adds [p. 93] to end of ist par. " and 

 I have seen one much larger inner whorl, imperfect. U.L., i, Whitby." 

 The comma ought evidently to stand after " larger," not after " whorl." 

 Omits " L.L., R. H. Bay." 



Remarks 



Stages, conch, serpenticone ; periphery, i ; ornament, 5*. 



The larger specimen noticed by Simjxson, 1884, is figured in Plate 

 XXIXb. 



Genus Porpoceras, nov. (Gen. p. r) ; family Dactyloidse, Hyatt, 1867. 

 Geological position presumably fibulatum zone. 



Result 



Porpoceras vortex, Si.mpson sp. 1855, Whitbian, [fibulatum zone], 

 near Whitby. 



