AEGOCERAS ARMATUM. 343 



striae are fine, numerous, undulating, and annular; and the aperture has a roundish form. 

 This is a much more smooth and elegant form than the typical Aeg. armatum, still it may be 

 specifically identical with it. The smaller diameter of the whorls, and the greater distance 

 the ribs are apart from each other, greatly diminish the number of the lateral spines, 

 which are longer and more slender than in the typical specimens of Aeff. armatum ; the lobe- 

 line is likewise less complicated, and the septa are also more distant from each other than 

 in fig. 6 ; the inner whorls are striated without ribs or spines. This is a most interesting 

 form of the group Armati ; and probably the discovery of other specimens may dis- 

 close some features in its form that may justify the distinction Simpson assigned to it. 



In Aegoceras armatum (the typical form figured in PI. XXVIII, figs. 1 and 3), in the 

 middle period of life, the septal chambers are shallow and the lobe-line extremely convo- 

 luted, so that it is very difficult to follow its contouring. Professor d'Orbigny sketched 

 a generalised diagram of this line from the specimen he possessed, whilst I have had the 

 lobe-line traced on one of my specimens, and an enlarged drawing made therefrom 

 (fig. 6), which shows that the true ramification of the lobe-line is much more complicated 

 than d'Orbigny's figure led us to suppose. The siphonal lobe is as wide and almost 

 as long as the principal lateral, and has on each side four lateral branches. The siphonal 

 saddle is much wider and longer than the principal lateral lobe, and is divided into two 

 unequal portions, the external of which is the largest and terminates in many folioles ; 

 and the internal portion has a similar termination. The principal lateral lobe is highly ornate, 

 and from the wide base two large branches proceed from each side of the stem, which 

 terminate in a long terminal foliated portion. The lateral saddle is small and narrow 

 and terminates in three folioles. The inferior lateral lobe is small and narrow, about one 

 fourth the size of the principal, it develops three lateral and one long terminal branch ; 

 the accessory lobes are oblique and much ramified, and the whole lobe-line forms a most 

 highly complicated contouring of foliations. 



Affinities and Differences. — I have separated the Armatum' from the Jatnesoni-zone 

 because it constitutes an excellent Ammonite horizon, with an interesting series of elegant 

 forms which range themselves around the original Sowerbyan type as a centre, and which 

 commenced and terminated their existence with the dawn of the Middle Lias ; they had, 

 therefore, it would appear, a very limited life in time. 



A great family likeness runs throughout the group, which requires accurate observa- 

 tion and a critical judgment to discover and define ; all the species have the whorls more 

 or less armed with spines, and with ribs and striae extending across the siphonal area ; 

 in most of the species only one row of spines is developed along the margin of this 

 area, whilst in others there are two distinct series of spines, as in Aeg. densinodum, 

 Quenst., and Aeg. brevispinum, Sowerby ; the septal chambers in all are very shallow, and 

 the lobe-line is likewise highly convoluted. 



Locality and Stratigrajjhical Position. — The fine specimens figured in Pis. XXVIII 

 and XXIX were obtained from the Aegoceras armatum-zone at Lyme Regis, and very 



