44S THE LIAS AMMONITES. 



Ammonites Candidas, d'Orb., ' Lias superieur,' but oddly enough the author has omitted to 

 give any description of his plate, nor does he make any allusion to it either in the text of 

 his great work ' Paleontologie Erancaise,' or in his ' Prodrome Paleont. ;' I believe, 

 however, in the identity of the Am. Candidas, d'Orb., with Harp, elegans, Young. 



The elegance of this Ammonite and the persistency of its beautiful lines long attracted 

 my attention, and I have collected many examples from the jet-rock and hard shale of the 

 Upper Lias near Whitby, where it was at one time a very common species. The specimen 

 I have figured is a very good type of this form. The shell is thin and very well 

 preserved in the figured specimen, which displays the structure of the radii in the shelly 

 laminpo itself, and the impressions they leave upon portions of the mould exposed to view. 



The shell is discoidal, compressed, and subcarinated. The keel does not form a 

 distinct prominent elevation, but is a protruding ridge rising from the middle of the siphonal 

 area, and is formed by the union of radii from the two sides joining together along the mesial 

 line ; the inner third of the whorl is slightly concave, and the outer two thirds convex and 

 sloping upward into the keel. Some authors describe this species as possessing a sharp, 

 prominent, entire keel, which does not agree with the foregoing description, so it is 

 right to state that my definition applies to specimens which have the shell-structure pre- 

 served entire, whilst in other specimens in which the mould only exists the keel is much 

 more distinct. The inner or spiral angle is acute and elevated (PI. LXIII, fig. 1), and 

 slopes inwards in a concave curve towards the penultimate whorl. The umbilicus, although 

 narrow, is much exposed by the successive slopes of the inner volutions of the spire. 



The shell thin, smooth, and shining, is ornamented on the sides in younger specimens 

 by moderately prominent sigmoidal ribs, which gradually become less defined in older 

 specimens, and finally degenerate into the very fine radii on the body-chamber. 



The aperture is narrow, elongated, and sagittate ; it has short, convex, lateral lappets, 

 and a prolonged abdominal spine, with a thickening of the shell around the aperture. 



The mould of this species, when entirely denuded of its shell, has a very smooth, 

 shining appearance, and the very characteristic concave sloping of the inner margin of 

 the whorls, already noted, is now more conspicuous, so that it is readily distinguished 

 from its congeneric associates by this character alone. 



Affinities and Differences. — It differs from Harp. Lythense in having narrower 

 whorls and smaller ribs in youth, and by wanting them in age. The sloping concave 

 character of the inner margin of the spire in Harp, elegans forms a marked contrast 

 to the acute spiral angle and vertical marginal wall of the umbilicus in Harp. Lythense. 



It differs from Harp, exaratum by the absence of the well-marked regular sigmoidal 

 ribs and deeply grooved valleys possessed by that species, and by the want of the concave 

 marginal slope of the whorls, which forms so good a diagnostic character; this also 

 distinguishes it from Harp. Lythense, and it is equally good as a diagnostic trait of 

 character between Harp, elegans and Harp, exaratum. 



Llarp. elegans is nearly related to Harp, ovatum ; the latter, however, has convex 



