92 INFERIOR OOLITE AMMONITES. 



smoothness of the test ; but they may readily be separated from it, as they possess a 

 solid instead of a hollow carina, a concave inner margin, and very different sutures. 

 Hi/perlioc. discoideum is, on account of its peculiar shape, labelled Am. Truellii in 

 some public and private collections. This is a very grave error, which this generic 

 subdivision of Ammonites will in the future, I hope, serve to obviate by really 

 emphasizing the great differences existing between such species of Ammonites, 

 and by drawing more particular attention to structural details. 



Hypeelioceras Walkeri, 8. Buckman. Plate XVI, figs. 1 — 11. 



Adult. — Discoidal, much compressed, strongly carinate ; whorls, very flat and 

 broad, with their sides almost exactly parallel (not swollen), ornamented merely 

 with very fine, rather strongly curved, sigmoidal lines of growth. Ventral area 

 much flattened, sharply defined, carrying a broad, prominent, solid carina, with 

 its outer edge rounded. Inner margin shallow, concave, fairly upright, but more 

 sloping in the neighbourhood of the body-chamber. Inclusion nearly four-fifths 

 of the preceding whorl, but less where the body-chamber comes. Umbilicus 

 small and shallow, formed of a series of small steps, mostly flat and smooth, only 

 the inner ones showing traces of small ribs. 



Immature. — Up to a diameter of about eight lines the whorls are ornamented 

 with straight, simple ribs, very slightly bent forward on the lateral area. At this 

 age the carina is well formed and distinct, and the ventral area flat and quite 

 smooth ; but the inner margin can scarcely be said to have appeared. Beyond 

 this size the ribs gradually change, becoming more and more subfalciform in 

 shape. At about seventeen lines in diameter they gradually fade away, giving 

 place to extremely fine lines of growth, which have more of a sigmoidal curve. 

 The carina is now large and trenchant showing no indication of any hollow space, 

 is much compressed laterally, and is slightly rounded on its outer edge. The 

 ventral area is narrow and flattened ; the sides of the whorls are very nearly 

 parallel, though slightly swollen about the middle. At a diameter of fourteen 

 lines we find the inclusion to be a trifle more than half the whorl (at an earlier 

 stage we ought to find it less), but at thirty lines we find that it is four fifths of 

 the preceding whorl. The termination is furnished with short lateral lappets in 

 the young (Plate XVI, fig. 7). 



Three varieties of this species have been depicted on Plate XVI ; and they 

 were chosen because they exhibited certain slight differences. Each form, when 

 adult, attained a diameter of about the size shown by fig. 1 ; but it was unnecessary 



