318 THE LIAS AMMONITES. 



Second Section. — Angulati. 



Aegoceras angulatum, Schlotheim. PI. XIV, figs. 5, 6; PI. XVII/ figs. 1 — 6. 



Ammonites angulatus, Schloth. Petrefactenkunde, p. 70, 1822. 



— Redcaeensis, Young Sf Bird. Geol. Surv. Yorksh. Coast, p. 258, pi. xiv, 



fig. 13, 1828. 



— ANGXJLIFERUS, Phillips. Geol. of the Yorkshire Coast, p. 192, vol. i, 



tab. 13, fig. 19, 1829. 



— coiiTJBKATUS, Zieten. Verstein. Wurtembergs, tab. 3, fig. I, p. 3, 1830. 



— ANRtJLATCS, Fon Buck. Ueber Ammoniten, p. 12, 1832. 



— Redcauensis, Simpson. Monograph on Ammonites of the Yorkshire Lias, 



p. 55, 1843. 



— ANGULATDS, Quenstedt. Cephalopoden, p. 74, pi. iv, fig. 2, 1846—9. 



— — Quenstedt. Petrefactenkunde, p. 354, tab. 27, fig- 7, 1852. 



— — Quenstedt. Der Jura, p. 43, tab. 3, fig. 1, 1858. 



— — Chapuis et Bewalque. Fossiles Terr. Second. Luxembourg, 



p. 36, pi. iv, fig. 1, 1853. 



— — Oppel. Juraformation, p. 75, 1856. 



— Eedcakensis, Simpson. Fossils Yorkshire Lias, p. 100, 18.55. 

 Aegoceras angulatum, Tate Sf Blake. Yorkshire Lias, p. 271, 1876. 



Diagnosis. — Young shell compressed; whorls involute, half enveloped; sides with 

 twenty-six to thirty, sharp, simple, flexed ribs, which commence at the umbilical 

 margin, pass round the outer border, and terminate abruptly in a furrow in the centre 

 of the siphon al area, or in a smooth truncated surface. 



Adult shell, several inches in diameter, compressed ; whorls flattened, sloping towards 

 the outer margin; sides smooth, with long, faint, biflexed ribs, and shorter and more 

 marked costae near the border ; area narrow, round, smooth ; aperture compressed. 



Dimensions. — One of my largest specimens from Lyme Regis measures 180 milli- 

 metres in diameter. Height of the last whorl near the mouth 65 millimetres; width 

 40 millimetres. Whorls one-half covered by the involution of the spire. 



Description. — Like many other Ammonites, this species must be studied at different 

 stages of growth, if we wish to understand its true characters. Up to about the 

 diameter of an inch the whorls have from twenty-five to thirty sharp simple ribs, which 

 terminate abruptly and form a marked angle on the back — hence the origin of the 

 specific name ; at a later period the ribs divide, become less distinct, and the angle on 

 the back disappears ; in advanced age the sides are smooth, the siphonal area is narrow, 

 and without ribs. 



1 See pages 322 and 323. 



