462 THE LIAS AMMONITES. 



Harpoceras bicarinatum, Miinster. PI. LXXXII, figs. 9, 10, 10«, 11. 



Ammonites bicarinatus, Miinster. In Zieten's Versteiner Wurtemberg, p. 21, pi. xv, 



fig. 9, 1830. 



— elegans, Sowerby. Mineral Conchology, vol. i, p. 213, pi. 94, 1815. 



— — Phillips. Geol. York., pi. xiii, fig. 12, 1829. 



— complanatus, cCOrhigny. Pale'ontol. Francaise, Terr. Jur., p. 353, pi. 1 14. 



figs. 1—4, 1842. 



— bicarinatus, Quenstedt. Der Jura, p. 5/8, 1858. 



— — Dumortier. Depot Jurassique, partie iv, p. 55, pi. xi, figs. 



3—7, 1874. 



Diagnosis. — Shell compressed ; whorls broad, sides flattened and ornamented with 

 simple, narrow, nearly equal, biflexed costse, separated by well-marked sulcations ; 

 siphonal area obtuse, with a small, central, vertical carina raised on a flat, and two 

 lateral ridges which bound the flanks and form therewith an acute angle ; umbilicus 

 narrow, with vertical walls ; aperture compressed, elongated, subquadrate. 



Dimensions. — Transverse diameter 49 millimetres ; width of umbilicus 6 millimetres j 

 height of last whorl 25 millimetres ; width of aperture 12 millimetres. 



Description. — This beautiful little Ammonite has occasioned much confusion in the 

 description of the " falcifers " of the Upper Lias beds, in consequence of d'Orbigny having 

 figured in his plate 114, fig. 3, a shell of Am. bicarinatus as the young shell of his Am. 

 complanatus. 



Harp, exaratum {complanatum, d'Orb.) has a round siphonal area and prominent 

 keel (PI. LXII, figs. 1 — 3), whilst Harp, bicarinatum has a flat area, small central keel, 

 and two lateral ridges separating the area from the flanks. I have a series of four fine 

 specimens from Milhaud, Aveyron, before me, which show this character very well. The 

 specimen I have figured (PL LXXXII, figs. 9, 10) is the only English example I know. 

 There is a neatness and angularity about the entire outline of this Ammonite which serve 

 to distinguish it. Dumortier put this question to a crucial test, he broke up several large 

 specimens of Am. complanatus, d'Orb., for the purpose of procuring their small interior 

 whorls in order to compare them with true forms of Am. bicarinatus. The result of this 

 comparative study convinced him that there was nothing to justify the union of the two 

 shells, and that Am. bicarinatus was a good species, well separated by the form of its carina 

 and whorls, which never varied, and that none of the examples of Am. complanatus, d'Orb., 

 which he had obtained had shown a structure of the siphonal area similar to the one so 

 accurately delineated in figs. 9 and 10, PI. LXXXII. 



The whorls are oval and the spire extremely involute, so that the last whorl occupies 

 one half the diameter of the disc ; the costae arise by simple, hair-like stems around 



