368 THE LIAS AMMONITES. 



Affinities and Differences. — In the young shell, Aeg. latmco&ta resembles Aeg. 

 capricornus, although the style of ribbing in the two shells is different ; Aeg. capricornus 

 has rounder whorls with simple obtuse ribs without spines. Compare PL XXXII, 

 fig. 1, with PI. XXXIV, fig. 1.^ In both species it is the sixth whorl which becomes 

 suddenly enlarged, Aeg. latcecosta becoming Aeg. Henleyi, and Aeg. capricornus evolving 

 Aeg. heterogenmn. No description can do justice to the remarkable morphological change 

 effected in the sixth whorl of their shells, and so I have given good figures of the best 

 examples I know, for it is very difficult to obtain a specimen of Aeg. heterogenum now on 

 the Yorkshire coast, and it is equally rare to find one of Henleyi at Charmouth; in fact, 

 the example I have figured in Plate XXXIII is the only one I know that shows the inner 

 and outer whorls in situ, as it lived in the Middle-Lias Sea. 



Locality and Stratigraphical Position. — I have obtained all my specimens of Aeg. 

 Henleyi from the Green Ammonite-bed, near Charmouth, where it is associated with 

 Aeg. Davcei, Aeg. BecJiei, Lytoceras fimbriatum, and Phylloceras Loscomhi ; for details 

 of the petrology of this remarkable bed the reader is referred to p. 89. A portion of 

 the shell is preserved on one of the whorls, where it is seen to be very thin, and many of 

 the fine striae which cross the area are only feebly impressed on the mould. In one large 

 specimen with six whorls the latcecosta condition continues up to a diameter of 130 

 millimetres, whilst in a dwarfed shell with six whorls it continues to 70 millimetres, from 

 which I infer that it is not the size of the shell, but the number of the whorls which 

 indicates the age of the Ammonite, and determines the excessive development of the body- 

 chamber of this remarkable Ammonite. Aeg. Henleyi appears to be a rare species in 

 Prance. The late Dr. Peynes, who made the study of Lias Ammonites a speciality, 

 knew only two specimens in the Aveyron, both of which had been collected from lime- 

 stones of the Middle Lias at St. Jeau-d'Alcapies in company with Lytoceras fimbriatum. 

 Sow., Aeg. Bechei, Sow., and Gryj^hcea cymbium, the same associates with which it lies 

 in the Green Ammonite-bed of the Dorsetshire coast. 



Aegoceras capricornus,^ ScMotJteim. PI. XXXIV, figs. 1 — 8. 



Ammonites capeicokkus, Schlotheim. Petrefaktenkunde, p. 71, 1820. 



— MACTJLATUS, Young and Bird. Geol. Survej', p. 248, pi. xiv, fig. 12, 



1822. 



— — Phillips. Geol. of Yorkshire, pi. xiii, fig. 11, 1829. 



— — Quenstedt. Cephalopoden, p. 85, tab. iv, fig. /, 1849. 



— — Simpson. York. Lias Fossils, p. 48, 1855. 



As to the synonymy of this Ammonite, I may observe that Dr. K. Schlonbach made 

 a very careful study of the Schlotheim Collection in the Mineralien-Kabinet of Berlin,. 

 1 In the explanation of PI. XXXIV this species is termed in error degoceras maculatum. 



