HARPOCERAS EXARATUM. 443 



Dumortier has figured and described an Ammonite under the name Am. ewarafus, 

 which is very different from Young's typical shell ; this is either a defaced specimen or 

 another form. The plates which I consider to contain good figures of Harp, exaratum are 

 those given by d'Orbigny, pi. 114 ; Dumortier pi. x and xi, figs. 1, 2 ; and the 

 medium-sized shell drawn in PI. LXII, figs. 1 — 3, in this work. The late Dr. Reyues in 

 his ' Monographic des Ammonites,' Marseille, 1879, pi. iv, "Ammonites du Lias superieur," 

 figs. 4, 5, 6, gave admirable delineations of this species under the false name Am. 

 degans. Sow. 



Affinities and Differences. — The Falciferi of Von Buch, whilst truly one of the most 

 natural families of the Amraonoida, is at the same time one of the most difficult to 

 differentiate into specific forms. Mere descriptions without carefully drawn and accurate 

 delineations are in a great measure valueless, hence the confusion which exists in 

 testing several of the species of this group. The difficulty an author experiences in 

 detecting mistakes is very great, unless he has a set of good type specimens to refer to for 

 comparison ; seeing so much depends on the shape of the whorl, the style and diameter of 

 the ribbing, the amount of involution, the inversion, eversion, or verticality of the spiral 

 angle, the structure of the keel, the contouring of the suture-line, and the form of the 

 appendages of the aperture ; and so closely balanced are all these points with each other 

 that without an actual comparison of typical specimens mistakes of identity are readily 

 made. 



Harpoceras exaratuvi closely resembles Harp, serpentinum in general outline, but 

 differs in the following details : — The sides are more convex, the whorl is higher, the 

 involution greater, the umbilicus narrower, and the sigmoidal valleys between the radii 

 are much more sharply defined ; the longitudinal sulcus also so characteristic of Harp, 

 serpentinum is absent. 



Harp, exaratum. differs from Harp, bicarinatum, Miinster ; in the latter the sides 

 are flatter, the area squarer, with a low keel and two lateral carinse on the boundaries of 

 the area ; the involution is likewise greater, and the radii broader, less numerous, and 

 very falciform (see PI. LXXXII, figs. 9, 10, 11). 



Harp, exaratum resembles Harp. Lythense ; but in the latter the outer whorl is 

 higher, the ribs fewer, larger, and rounder ; the umbilicus narrow, the spiral angle very 

 acute, and the sides of the walls vertical, exposing all the inner volutions ; the aperture 

 is sagittate, the upper portion sloping away toward the keel. 



Locality and Stratigraphical Position. — This fine Ammonite, which sometimes attains 

 210 millimetres in diameter, is found in the zone of Harp, serpientinum, Upper Lias, at 

 Whitby, Yorkshire. 



