443 THE LIAS AMMONITES. 



sometimes found. Confusion has been introduced into the history of this Ammonite in 

 consequence of d'Orbigny having mistaken this shell for Ammonites complanatus, Bruguiere, 

 which is a different species and not well defined by its author "Ammonites striis 

 simplicibtis undulatis ohliquis, sulco utrinque intermedia carina acuta," Bruguiere, 

 ' Encycl. Method.,' torn, i, p. 38. This diagnosis applies equally to two distinct forms ; 

 a, those in which the keel is not separated from the sides, and forms only the 

 continuation thereof, and b, those in which the area shows three carinse with two 

 intermediate sulci. The first form has Harp, ecearatum for its type, the second has 

 Am. hicarinatus, Miinster (in Zieten), for its type. D'Orbigny's pi. 114, figs. 1, 2 is an 

 admirable delineation of Ilarpoceras exaratum and perfectly in accordance with a very 

 good figure given by Chapuis and Dewalque under the name Am. complanatus. In 

 Harp, exaratum the shell is discoidal, depressed, carinated, and extremely involute, the 

 volutions concealing three fourths of the whorls, and the height of the outer whorl about 

 one half the diameter of the disc ; the radii are numerous, always biflexed and simple, 

 appearing as if etched out with a graver, as expressed by the name, and extended upon 

 the keel, which is not separated from the sides, of which it is only a continuation, and 

 upon which they are prolonged in a marked manner. The very numerous radii parting 

 from the spiral junction first incline forward, then bend backward, and again sweep 

 forward in such a manner as to describe a sickle-shaped curvature ; and, as they are all 

 simple, it follows that the radii stand wide apart around the siphonal area, and are 

 closely set together in a very regular form near the spiral angle (PI. LXII, fig. 1). 

 The umbilicus is narrow and deep with an angular margin without being trenchant, and 

 the inner edge of each whorl overhangs part of the next whorl ; this form of the spire 

 permits a much larger portion of the inner volutions to become visible than would have 

 been the case had the margin inclined outwards instead of inwards. The overhanging 

 of the whorls constitutes a difference between Harp, exaratum and Harp, bicarinatum. 

 In Harp, bicarinatum the inner sides of the whorls are vertical or nearly so. 



The evolution of Harp, exaratum is well shown in two or three specimens collected by 

 me in one mass from the jet-rock. One of the shells is only 21 millimetres in diameter, 

 yet the sides are well ribbed with prominent radii, and there is a small carina on the 

 siphonal area ; the proportions and ornamentation of the young shell are clearly similar 

 to those of the larger specimen found with it in the same mass of stone. 



D'Orbigny united with his Am. complanatus the Am. bicarinatus, Zieten, and stated 

 (p. 355) that the young shells of Am. complanatus show a tricarinate siphonal area, and he 

 figured in his pi. 114, fig. 3, a small Am. bicarinatus in support of his assertion. In this 

 the learned author was mistaken, as the young shell of Harp, exaratum, which is identical 

 with his Am. complanatus, has only a single carina in the area. In fact. Harp, bicarinatum 

 is a distinct form, for from its flattened sides and square area two small lateral carinas 

 arise on each side of the keel ; three specimens before me from Milhaud, Aveyron, 

 support this determination. 



