178 



Dimensions. — Transverse diameter of a large specimen 8 inches ; height 

 of the aperture 1^ inches ; width 1-/^ inches. 



Description. — This very distinct and characteristic Ammonite of the 

 Lower Lias is readily distinguished by the round back and bi-tuber- 

 culated ribs. 



The whorls increase so slowly in diameter, that in a large specimen 

 before me, eight inches in diameter, there are seven turns of the shell ; 

 the sides of the whorls are convex, and slightly flattened in the middle, 

 they have from 32 to 34 thick, straight, lateral costse, with two tubercles 

 developed on each, the outer is the largest, and in some specimens 

 becomes spinous ; the inner is smaller and more obtuse, and the costse 

 disappear near its inner border. 



The back is round, and in some specimens three or four fine transverse 

 bands extend across the back from one outer tubercle to the other, and 

 fine transverse lines of growth are seen on the shell figured in PI. 1, fig. 3. 



The spire is fonned of very narrow whorls, which are slightly involute 

 and ftilly exposed. 



The aperture is round above, flattened on the sides, and very little 

 diminished by the turn of the spire. 



The septa are symmetrical, and have an extremely complicated 

 aiTangement of lobes and saddles, of these I am only able to figure a 

 portion. The dorsal lobe is nearly as long as the laterals, fig. 3, a, it 

 is deeply bifid, and on each side thereof are six or seven long narrow 

 digitations. The superior and inferior lateral lobes have an exceedingly 

 complex ari*angement in the distribution of their side branches, this 

 will be much better understood from the excellent figure I have given 

 of them than by any verbal description thereof The saddles consist 

 of mimerous slender foliations. 



This species preserves its specific form through life, with only a very 

 limited variation ; the fry, up to half an inch in diameter, are smooth, 

 the outer tubercles are first developed, and the inner ones soon follow, 

 so that when the shell attains one inch in diameter, it exliibits the 

 distinguishing characters of the species, which are steadily preserved in 

 my largest specimen, eight inches in diameter. 



Affinities and Differences. — This Ammonite is very distinct from all 

 its congeners in the Lower Lias, and is easily identified by its roimd 

 back, slow increase in the diameter of the whorls, the bi-tuberculated 

 character of its numerous costae, and complex i-amifications of its septa, 

 — by these charactei-s it is easily distinguished from them. 



A. Valdani, d'Orb., of the Middle Lias, has likewise bi-tuberculated 

 costae, but the flatness of this shell, and the elevated median ridge on its 



