4b YORKSHIRE TYPE AMMONITES Dec. 



4. AMMONITES MULGRAVIUS, Young & Bird 

 (Plates IVa, IVb) 



Original Description 

 [Young & Bird, 1822, p. 251.] 



" No. 8, PI. XIII, greatly resembles this last shell [a. elegans of 

 Sowerby], and has the same locahties ; but the internal angles of the 

 aperture or spire, instead of being truncate or bevelled, are sharp and 

 rather prominent, the margin of the outer whirl projecting somewhat 

 over the next whirl. There is also a groove, as in a. Hildensis, generally 

 dividing the whirl longitudinally, or rather spirally. This sheh is often 

 pyritous, and of great beauty. As it occurs chiefly on the shores of the 

 Mulgrave estates, we may name it a. Mulgravius." 



Additional Details 



Young & Bird, 1828, p. 266. " No. 8, PI. xiii, is a very handsome 

 shell of this family [ammonites with sigmoidal ribs], with ribs approaching 

 to the hook shape, and a sharp keel. The ribs are broader than the 

 spaces between them ; and these spaces are often like deep furrows, 

 grooved out between the ribs. The aperture is somewhat sagittate ; 

 and the inner edge of each whirl overhangs part of the next whirl. This 

 noble ammonite, which sometimes exceeds 10 inches in diameter, we 

 have named A. Mulgravius, as we found it first on the shores of the 

 Mulgrave estates, in has bands." 



[P. 267.] " All these sigmoidal ammonites are keeled. Some of 

 them have often a depressed line in the side of each whirl ; particularly 

 A. Mulgravius. The chambers of this last sometimes contain mineral 

 oil : they are also found lined with pearl spar, and containing here and 

 there large crystals of calcareous spar." 



Remarks 



Stages, conch, between serpenticone and oxycone ; periphery, 3c ; 

 ornament, 4c. 



For evidence as to size of the type-specimen and its amount of 

 umbilication see A. exaratus, p. 5b: Young & Bird's figure is much 

 reduced, and the drawing of the umbihcus quite misleading. 



The specimen has a septicarina ; and there is at least half a whorl 

 of body-chamber. The inner whorls show swellings — the stage of the 

 furcating ribs, which precedes the stage of single falciforms. 



The genus is Harpoceras, Waagen (Gen. p. 7) — family 

 Hildoceratidae ; and the geological position, according to Simpson, 

 (1884, 109, xii) is Upper Lias, 5b. 



Result 



Harpoceras mulgravium. Young & Bird, sp., 1822, Whitbian, 

 falcifenim-zone, Mulgrave, about 6 miles N.W. of Whitby. 



