334 THE LIAS AMMONITES. 



points ; and when the shell has reached 35 millimetres in diameter, the lateral bifurcated 

 expansions of the ribs impart marked features to the lateral figure of the shell. 



Dimensions. — Transverse diameter 24 millimetres ; width of umbilicus 12 millimetres ; 

 height of aperture 5 millimetres ; transverse width 5 millimetres. 



Description. — Professor Quenstedt first recognised this very interesting Ammonite 

 in the Lower Lias of Balingen and Ofterdingen, Wiirtemberg. It presents an assemblage 

 of characters at different stages of growth which renders it a most interesting morpho- 

 logical study. In the first four whorls, and when the diameter of the tube does not 

 exceed that of a crow's quill, the shell is smooth and without ribs ; and in this phase of 

 growth it has a circular mouth-opening, and only fine striae, scarcely visible without a lens, 

 are seen in the siphonal area covering the smooth shell and bent towards the aperture. 

 In the next stage of growth it develops fifteen short, stout, lateral ribs, which disappear 

 from the siphonal area ; and after another whorl or so of growth the ribs extend outwards 

 in thin, sharp folds, which shoot forth two spinous processes. The shell has now attained 

 the size of a florin, and in this phase of growth the sides are ornamented vidth eighteen 

 thin, sharp, compressed ribs, each terminating in a spinous process (PI. XXVI, fig, 4), 

 of which the outer spine is the largest ; in this stage of growth the aperture is much 

 wider in its lateral than in its vertical diameter. The shell is nearly evolute, and 

 in consequence the winding of the spire it is sometimes excentral, and then assumes the 

 obliquity of a Turrilite ; from Captain Valdan finding some specimens in this 

 condition in the Lower Lias of Augy-sur-Aubois, near St. Amand (Cher), with a spiral 

 dextral shell, d'Orbigny regarded them as Turrilites, and figured them as such in the 

 ' Paleontologie Ejaufaise ' (pi. 42, figs. 1 — 3), an error which is repeated in another evolute 

 shell, the Aegoceras Coynarti. Both species were collected from the marls of the Lower 

 Lias with Gryplicea arcuata, A similar error has been committed in mistaking the evolute 

 Arietites raricosfatus for a Turrilite, and figuring it as Turrilites Bohlayi. The lobe-line is 

 simple and is very well seen in one of my young specimens ; the siphonal lobe is the 

 longest and terminates in projecting processes (fig. 2) ; the siphonal saddle is wide, with 

 rounded denticles ; the lateral lobes are both about the same size and figure, and the 

 lateral saddles are larger than the lobes ; the auxiliary lobes and corresponding saddles 

 are too much concealed to admit of accurate description, the type, however, of the entire 

 lobe-line is that of the character of a Capricorn Aegoceras in all its essential details. 



Affinities and Differences. — Aegoceras biferum in early youth resembles the young of 

 Arietites semicostatus, being like it smooth and destitute of ribs. In its second or ribbed 

 phase it resembles a young form of Aeg. Capricornus with lateral ribs and a smooth 

 siphonal area ; in its third stage, when it has acquired its winged ribs with compressed 

 bi-spinous terminations, it presents a form unique amongst the group to which it belongs. 



Locality and Stratigraphical Position. — This is a very rare Ammonite. I have 

 never seen any other examples in England except the few specimens I collected from 

 the Great-Western Railway cutting in the Lower Lias Beds of Lansdown Road, 



