73 b YORKSHIRE TYPE AMMONITES— II May 



73. AMMONITES LURIDUS, Simpson 

 (Plate LXXIII) 



Original Description 



" 39. A[mmonites] luridus. [M. Simpson, 1855, p. 46.] 



["I. Without a dorsal keel or furrow. 



"a. No spines, p. 35.] 



" Thickish ; volutions 5, exposed, outer whorl more than J the 

 diameter ; radii sharpish, strong, annular, rather depressed on the back, 

 where they bend towards the aperture, separated by widish spaces ; 

 a few annular striae ; aperture little more than a semi-circle ; diameter 

 if inch." 



" The whorls of this ammonite diminish very little in width, forming 

 a shallow umbilicus. The shell is thick, and formed of several layers, 

 of a brown colour. — Mr. Leckenby's Col." 



Simpson, 1884, 76, the same. 



46} 



45 I 



Remarks 



Proportions : J 50 27 34 



1 27 30 48 



Subplaty-, perpachy- to pachygyral, perlatumbilicate. 



Stages, conch, serpenticone ; periphery, 1 ; ornament 4c with 



suggestion of (obsolescent ?) 5*. The species is presumably in the 



post-tuberculate costate stage : the single lateral costae shew a suspicion 



of tuberculation on the latero-peripheral edge, — the tubercle can hardly 



be seen, but it can be felt with the finger. 



The species must be a cadicone in youth, up to about 6 mm. diameter, 

 the whorl being depressed, stout, the umbilicus narrow, deep. Later, 

 the umbilicus flattens out, while increase in actual thickness of whorl 

 is slow (proportionately, the decrease in the last whorl is 14 per cent.) 

 On the periphery the primary costae break up into 3 or 4 very obscure 

 secondary costae, the first of which has only a slight forward sweep, 

 the next has more curvature, and the last of the bunch has the most. 

 The breaking up begins on edge of periphery, but sometimes a little 

 earlier. 



The specimen depicted has the body-chamber almost complete, 

 about half a whorl. The suture-line is noticeable for the distinct 

 A character of the cell parting the peripheral or external lobe. The 

 inner part of the suture-line is not retracted. 



Genus, Beaniceras, nov. (II, Hi) ; family Liparoceratidae. Geological 

 position doubtful : the deep brown colour of the test is unusual for 

 Yorkshire specimens. Blake, 1876, 281, placing it as a synonym of 

 A eg. capricornum, indicates his opinion that it occurs towards the top 

 of the Charmouthian. A specimen from Stow-on-the-Wold, Gloucester- 

 shire, (purchased), is from ochraceous Lias ; specimens of other species 

 of the genus are from yellow Lias of [Hewletts' Hill, near] Cheltenham, 

 about striatum zone. 



For this species and homceomorphy see II, vi, Hi. 



Result 



Beaniceras luridum, Simpson sp. 1855, [Charmouthian, circa 

 striatum zone ? near Whitby]. 



