370 



THE LIAS AMMONITES 



Aegoceras heterogenum, 



and Bird. PI. XXXV, figs. 4— G; PI. XXXVI, figs. 

 1—4. 



Ammonites hetekogenes, Young and Bird. Geol. Surv., p. 264, pi. xiv, fig. 7, 1828. 



— HYBBiDA, d'Orhigny. Pal^ontol. Fran?. Terr. Jurass., t. i, p. 285, 



pi. 85, 1842. 



— — Oppel. Mittlere Lias Schwabens, Jahr. WiJrt., p. 53, pi. iii, 



fig. 3, 1853. 



— HETEROGENES, Simpson. Fossils York. Lias, p. 69, 1855. 



Diagnosis. — Shell irregular in form ; whorls six, all exposed ; inner whorls small, 

 round, and slightly involute, with annular obtuse ribs ; body-chamber greatly enlarged, 

 forming the outer whorl ; ribs more closely approximated, and having two rows of 

 tubercles developed on each ; from the outer marginal tubercle the ribs split up into two 

 or three divisions, and cover the convex siphonal area with fine transverse striae; 

 aperture wide, oblong ; lobe-line highly complicated. 



Dimensions. — Transverse diameter 125 millimetres; width of the umbilicus 50 milli- 

 metres ; height of the body-chamber 50 millimetres ; height of penultimate whorl 20 

 millimetres; height of aperture 52 miUimetres ; width 50 millimetres. 



Description. — This is another very remarkable Ammonite closely related to Aeg. 

 Henleyi^ like it the body-chamber in the sixth whorl becomes much expanded, and 

 assumes a form and livery widely different from that exhibited in early life. It was 

 well remarked of this species by the Rev. George Young, when he proposed the species, 

 " that it is one of the most singular of all our Ammonites, the outer whorl of which has 

 also two rows of knobs. The interior part of the shell is comparatively flat, with ribs 

 rather prominent and flattened on the back, very much like those of Am. maculatus ; near 

 the outer whorl the ribs begin to have two slight knobs on the sides, and on that whorl 

 the ribs grow depressed and the knobs elevated, making two prominent rows, as is some- 

 times the case in the outer whorls of the Am. perarmatus in the Oolite formerly noticed. 

 But the most remarkable circumstance to be stated is, that the last part of the outer 

 whorl suddenly swells to a great thickness, as if it had belonged to another shell ; the 

 diSerence being the more striking, as the ribs in this part, instead of being flattened on 

 the back, are split into three at the outer row of the knobs. The mouth, as in the last 

 species, is sub-heptangular. We may name this singular shell Am. heterogenes^ 



PI. XXXV, fig. 4, shows the side view of a small specimen. Here the two rows of 

 tubercles are very prominent, the inner row being at some distance from the spiral suture, 

 and the outer row near the margin of the area ; the intervening ribs, fig. 5, show the 

 front view of the tubercles : we observe also how the obtuse annular ribs are set widely 

 apart on a whorl of moderate width with tubercles developed on the margin of the round area. 

 In fig. 6 we note the sudden expansion of the body-chamber, the width of the siphonal 



