AMALTHEUS DISCOIDES. 467 



Harpoceras discoides, Zieten. PI. LXXXII, figs. 12, 12«, 13. 



Ammonites discoides, Zieten. Verst. Wurts., p. 21, pi. xvi, fig. 1, 1830. 



— — d'Orbigny. Paleontol. Francaise, Ter. Jurass., p. 356, pi. 115> 



1842. 



— — Oppel. Die Juraformation, p. 245, 1856. 



— — Quenstedt. Der Jura, p. 283, pi. 40, fig. 7, 1858. 



Diagnosis. — Shell compressed, carinated ; whorls broad, nearly entirely involute 

 sides compressed, slightly convex, and covered with bitiexed sulcated undulations 

 umbilicus nearly occluded ; siphonal area narrow, lanceolate, and slightly serrated 

 aperture very deep, broad at the base, and narrowly lanceolate at the outer border. 



Dimensions. — Transverse diameter 70 millimetres ; height of the last whorl at 

 aperture 38 millimetres; transverse width at thickest part 15 millimetres; height of 

 aperture 35 millimetres; width of umbilicus 5 millimetres. 



Description. — This is a very rare Ammonite in the Cephalopoda-bed of Frocester, 

 and the specimen I have figured is the largest I have collected ; it agrees so well with 

 Zieten and d'Orbigny's figures that there is no doubt about the identity of the species. 



The shell is much compressed, strongly carinated, and provided with a very 

 small keel. The whorls are broad and the last exceeds in depth more than half the 

 diameter of the disk, so that the shell is extremely involute with a very small umbilicus. 

 The sides are ornamented transversely with simple, equal, biflexed costae, which spring 

 separately from the circumference of the umbilicus, and gently and gracefully bend 

 forward, then incline backwards, and afterward shoot forward towards the outer border, 

 the costae are separated by shallow sulci which have a similar curve to the costae (PI. 

 LXXXII, fig. 12). 



The siphonal area is narrowly lanceolate, and provided with a short and slightly 

 prominent keel. The spire is composed of compressed whorls which are angular towards 

 the umbilicus and trenchant at the border, the umbilicus is very narrow with declining 

 walls. The aperture forms an acute angle, rounded at the base (fig. 13). The suture- 

 line is very complicated, and as none of my specimens show the beautiful lines of this 

 structure, I have copied d'Orbigny's capital figure of the same. The chambers are 

 symmetrical and divided on each side into nine lobes and nine saddles formed of single 

 parts. 



The siphonal lobe nearly as long and much wider than the principal lateral lobe, is 

 provided with four branches ; of these the inferior branch is very large and oblique. 

 The siphonal saddle, twice as large as the principal lateral, is divided into two large 

 unequal-sized branches, of which the internal is the widest, with a very large 



