AMALTHEUS GUIBALIANUS. 385 



generally few or no undulations, while they are more distinct in the centre, which, if even 

 separated, might be mistaken for another species." 



The younger shells of this species, according to von Hauer, differ much from the 

 old form figured PI. XLIV. In a shell of from four to five inches in diameter, a distinct, 

 extremely thick keel, was found upon the middle line of the siphonal area, which in larger 

 specimens, about eight and a half inches in diameter, had disappeared. Younger shells 

 have the sides of the whorls covered with numerous straight ribs, which bend forward 

 towards the aperture as they approach the margin of the area. The ribbing of the inner 

 whorls is very well seen in the umbilicus of our figure. The outer whorl is very large 

 and capacious ; the sides are convex and ornamented with obscure undulations repre- 

 senting ribs, which, now becoming broad folds, disappear in old age. 



Von Hauer has given two very good figures showing the lobe-Iine of this Ammonite. 

 The siphonal lobe of a large specimen has two expanded branches, each having several 

 lateral digitations. The siphonal saddle is wide and terminates in six folioles arranged 

 almost in pairs ; the principal lateral lobe is longer and narrower than the siphonal ; it has 

 three branches on each side and a projecting terminal one ; the lateral saddle is wide and 

 deep, and terminates in a bifid and trifid and several lateral folioles ; the lateral lobe is 

 smaller than the principal ; it has two lateral branches on each side, and a pointed terminal 

 one ; the auxiliary lobes and saddles become much smaller between the inner lateral lobe 

 and the spiral suture. 



Affinities and Differences. — Specimens from six to eight inches in diameter resemble 

 Amaltheus Guihalianus in the general form of their shells and in the presence of a keel. 

 In larger shells the resemblance between these species diminishes and the adult conditions 

 of both are widely distinct from each other. 



Locality and Stratigraphical Position. — The figured specimen has no locality 

 attached to it, but the rock appears to resemble the Lias of Lyme. I have seen two 

 fine large specimens of this Ammonite at Charmouth. Both of these came from the 

 Lower Lias. As the collector in whose possession they were had not brought any other 

 shells from the bed I was unable to determine the horizon. Two large Ammonites 

 referred to this species were obtained from the Amal. owynotus-heds of Robin Hood's 

 Bay, and are recorded with doubt by Professor Blake as representing Amal. Greenhoughi. 



Monsieur Dumortier recognised some fragments of this Ammonite at Saint Fortunat, 

 Rhone ; Nolay, Cote-d'Or, in beds belonging to the zone of Amaltheus oxynotus. 



Amaltheus Guibalianus d'0rbig7iy. PI. XLV, figs. 1 — 7. 



Ammonites Guibalianus, d'Orbigny. Paleontol. Frangaise Terr. Jurasa., i, p. 259, 



pi. 73, 1842. 



— — Quenstedt. Leonard aud Bronu, Jahrbucb, p. 89, 1845. 



— — — Cepbalopoden,i, 351, 1849. 



50 



