79b YORKSHIRE TYPE AMMONITES— II August 



79. AMMONITES LATESCENS, Simpson 

 (Plate LXXIX) 



Original Description 

 " 106. A[mmonites] latescens. [M. Simpson, 1843, pp. 54, 55.] 



["III. Keel between two furrows." p. 48. 

 " b. Furrows distinct." p. 50.] 



" Depressed ; volutions 4 or 5, inner ones J concealed, outer whorl 

 f the diameter, sides regularly rounded ; radii waving, the last bend is 

 towards the aperture, obtuse, distinct on the outer margin ; keel sharpish, 

 [p. 55] entire, furrows on either side distinct ; aperture ovate ; diameter 

 1 J inch. 



" The breadth of the whorls, the regular convexity of their sides, 

 and the form of the radii, give this ammonite a strong resemblance to 

 A. striatulus, but it is destitute of striae, and has a distinct furrow on 

 either side of the keel." 



Additional Details 



Simpson, 1855, 100, adds after " outer margin," " on the inner 

 whorls distant and nearly obsolete." Under A. pinguis, p. 100, he 

 remarks "in A. latescens it [the keel] remains perfect where the shell 

 has been removed " (See No. 80). 



Simpson, 1884, 140, as in 1855. 



Remarks 



Proportions (without carina), 39, 28, 32, 42 ; subplaty-, sub- 

 pachygyral, latumbilicate. 



Stages, conch, serpenticone ; periphery, 5 ; ornament, 4(0 ?). 



There is a smooth stage till about 5 mm. diameter ; then some 

 obscure ribbing ; later, strong regular costae. The ribs of the early 

 whorls begin at the whorl- junction, later they commence about on the 

 edge, sometimes rather earlier, of a rounded umbilical margin. The 

 whorls are convex. 



What Simpson called the keel is only the impression of the partition- 

 band : the true keel is lost, for the specimen is mostly a cast, though 

 test remains between ribs. By ' keel sharpish ' Simpson may have 

 meant ' rather sharply defined,' which the flat impression of the partition- 

 band is : this " remains perfect where the shell has been removed." 



The specimen does not accord with Simpson's dimensions, being 

 nearer ij than ij inch ; the inner whorls are not \ but only about J 

 concealed ; the outer whorl is much less than f the diameter. Otherwise 

 the description fits well. 



Genus, Pseudogrammoceras, S. Buckman, 1901, 266 ; Mon. 1904, 

 cxliii ; and it belongs to the subfalciradiate section of the genus (Mon. 

 p. cxlix) ; family Hildoceratidae. The suture-line (see fig. 3), imperfectly 

 shown, is characteristic of the genus, L 2 being unequalsided, S 2 being 

 shallow. 



Geological position, the striatulus shales in a wide sense— the little 

 lamellibranch, Monotis substriata, Zieten sp., is characteristic. The date 

 may be somewhat later than striatulus hemera. 



Result 



Pseudogrammoceras latescens, Simpson sp. 1843, Yeovilian, 

 [struckmanni hemera, Peak, near Whitby.] 



