AMALTHEUS IBEX. 395 



Amaltheijs ibex, Quenstedt. PL XXXIX, figs. 4, 5. 



Ammonites ibex, Quenstedt. Fldzgebirge Wiirtembergs, p. 179, 1843. 



— BoBhfLYm,d'Orbiffny. Paleont. Fran9aise Terr. Jurass., p. 251, tab. 69, 



lfe42. 



— — Buckman. Murchison's Geology of Cheltenham, pi. xii, fig. 1, 



p. 89, 1845. 



— IBEX, Quenstedt. Cephalopoden, p. 101, tab. 6, fig. 6, 1849. 



— — Oppel. Mittlere Lias, Jahr. Wiirt., p. 87, t. 2, fig. 7, 1853. 



— BoBLAXEi, Morris. Catalogue of British Fossils, p. 290, 1854. 



— IBEX, Quenstedt. Der Jura, p. 1 19, tab. xiv, fig. 5, 1858. 



Diagnosis. — Shell discoidal, compressed ; whorls two thu'ds involute, flattened on 

 the sides, and ornamented with twenty-five to thirty short, wavy folds, which vanish at 

 about two thirds the height of the whorl ; siphonal area narrow, with twenty-five thick 

 transverse nodules, the apparent termination of the bi-flexed lateral folds ; aperture 

 oblong, truncated anteriorly. 



Dimensions. — Transverse diameter SO millimetres; width of umbilicus 15 milli- 

 metres ; height of aperture 40 millimetres ; width 1 7 milHmetres. 



Description. — This very beautiful Ammonite is seldom well preserved, it is in general 

 found in nodules of the Middle Lias, sometimes with its shell in a semi-fossil condition, 

 and oftener in the form of moulds with a well defined sculpture thereon. The shell is 

 discoidal and compressed ; the volutions, five or six in number, are extremely involute, 

 and the last whorl forms the conspicuous part of the disc ; its sides are flattened, 

 slightly convex, and ornamented with a number of bi-flexed folds or ribs, which are 

 more pronounced in middle-aged shells like fig. 5, which shows a specimen in my 

 collection magnified ; here the ribs consist of two sets, the inner series occupy two thirds 

 of the whorl, they are all flexed obliquely forward, and terminate abruptly in a thickened 

 portion ; the second series commence near the outer third, and pass very obliquely toward 

 the margin where they form thick knobs which extend transversely across the siphonal 

 area; between the series are found free indistinct ribbon-like impressions. In older 

 shells, as in the specimen (fig. 4), the ribs on the middle of the volution are not so pro- 

 minent, and gradually diminish on the body-chamber. The siphonal area is very narrow, 

 and the obtuse knobs, developed at intervals in this region, resulting from the outer series 

 of ribs impart a striking feature to the shell of this species, and liken it to the horn of 

 an Ibex. The aperture is oblong and flattened on the sides ; and the volutions of the 

 inner whorls, from the narrowness of the umbilicus, can only be seen in young specimens. 

 The lobe-line is pecuhar, and has been well described and figured by d'Orbigny. As none 

 of my specimens show this structure I quote his description. 



" The siphonal lobe is as wide and much shorter than the principal lateral lobe, and 



