AEGOCERAS BIFERUM. 333 



siphonal lobe is nearly as long as the principal lateral (fig. 7), it is deeply bifid, and on 

 each side thereof are six or seven long narrow digitations. The principal and inferior lateral 

 lobes have an exceedingly complex arrangement in the distribution of their side branches ; 

 this vfiW be much better understood from the excellent figure given of them than by any 

 verbal description. The saddles consist of numerous slender foliations. 



This species preserves its specific form through Hfe, and undergoes few morphological 

 changes, with only a very hmited variation ; the young shells (PI. XXXII, fig. 8), up to 

 1 3 millimetres in diameter, are smooth ; the outer tubercles are first developed, and the 

 inner ones soon follow, so that when the shell attains 25 millimetres in diameter it exhibits 

 the distinguishing characters of the species, which are closely preserved in the largest 

 specimen, 200 millimetres in diameter. 



Affinities and Differences. — This Ammonite is very distinct from all its congeners in 

 the Lower Lias, and is easily identified by its round siphonal area, slow increase in the 

 diameter of the whorls, the bi-tuberculated character of its numerous ribs, and complex 

 ramification of the lobe-line ; by these characters it is easily distinguished from them. 



Aeff. Valdani, d'Orb., of the Middle Lias, has likewise bi-tuberculate costse, but the 

 flatness of this shell and the elevated median ridge on its back, with the simpler character 

 of its lobes, and the great size of its saddles, serve to distinguish it from Aeff. JBircJni. 



Locality and StratigrapUcal Position. — The finest specimens of this shell are found 

 near Charmouth and Lyme Regis, in the zone o[ Arietites obtusus, associated with Arietites 

 Brookii on the same slab, and with the large Saurians, Iclithyosaurus platyodon and 

 Ichthyosaurus intermedins. It is rare in Gloucestershire, although I have several specimens 

 vphich vFere collected from the railway-cutting near Bredon ; one of these specimens showed 

 the arrangement of the septa figured at PI. XXXII, fig. 7. 



Aegoceras BIFERUM, Quenstedt. PI. XXVI, figs. 1 — 4. 



Ammonites bifer, Quenstedt. Flozgebirge Wurtembergs, p. 160, 1843. 



— — — Cephalopoden, p. 83, tab. iv, fig. 14, 1849. 



— — — Petrefaktenkunde,p. 356,tab. xxvii, fig. 20, 1852. 

 TuKEiLiTES Valdani, d'Orbigny. Pal. Fran(^aise; Ter. Jurassiques, t. i,p. 179, pi. xlii, 



figs. 1—3, 1842. 

 Ammonites bifek, Quenstedt. Der Jura, p. 103, tab. xiii, figs. 11—13, 1858. 



— — Emerson. Die Liasmulde von Markoldendorf ; Zeitsch. 



Deutscb. geol. Gesell., p. 327, pi. x, fig. 1, 1870. 

 Aegoceeas bifeeum, JNeumayr. Die Ammoniteu ; Zeitsch. der Deutscb. geologisch. 



Gesellschaft, p. 906, 1875. 



Diaynosis. — In youth shell smooth ; ai about the fourth whorl short, straight, blunt 

 ribs are developed with spines and processes; when it attains a diameter of 18 miUimetres 

 the ribs project, become thin, and winged, and the thin expansion terminates in two 



