AEGOCERAS PETTOS. 363 



Fourth Section. — Involuti. 

 Aegoceras PETTOS, Quenstedt. PI. XXXVII, figs. 5 — 7, PI. LXIX, figs. 5—6. 



Ammonites CRENATUs, Zieten. Versteinerung. Wiirttembergs, p. 1, tab. i, fig. 4, 



1830. 



— PETTOS, Quenstedt. Flozgebirge Wurtembergs, p. 1/8, 1843. 



— Geenouillouxi, d'Orbigny. Paleontol. Francaise; Terr. Jurassique, tome i, 



p. 307, pi. 96, 1842. 



— PETTOS, Quenstedt. Die Cephalopod., p. 179, tab. 14, fig. 8, 1849. 



— — Oppel. Mittlere Lias Schwab., Jahr. Wurtt., p. 93, 1853. 



— — Oppel. Die Juraformation, p. 165, 1856. 



— — Schl'unbach. Eisenstein d. Mittl. Lias, Zeitsch. Deutscb. 



geol. Gesell., p. 527, 1863. 



— — Beesley. Lias of Fenny Compton, Proceedings Warwick. 



Nat. Field Club, p. 16, 1877. 

 Aegoceras — Neumayr. Zeitscbrift der Deutsch. geol. Gesellsch., 



p. 906, 1875. 



— Grenouillouxi, Tate and Blake. Yorkshire Lias, p. 280, 1876. 



Diagnosis. — Shell discoidal, depressed ; umbilicus narrow, deep ; whorls half invo- 

 lute, sides ornamented with twenty-five short, oblique, mucronated ribs ; from each 

 tubercle three transverse striae proceed across the siphonal area, which is wide, depressed, 

 slightly convex, and without a carina ; aperture depressed, narrow in the vertical, wider 

 in the transverse diameter. 



Dimensions. — Transverse diameter 33 millimetres ; width of umbilicus 15 millimetres ; 

 height of aperture 8 millimetres ; transverse diameter 15 millimetres ; amount of involution 

 one half the height of the whorl. 



Description. — This is a very rare fossil in the Middle Lias of England, although a 

 very common Ammonite in the Numismalismergel of Germany, and of the Lias moyen 

 of France. The shell is discoidal, compressed, and with a very deep umbilicus ; from 

 the spiral suture a series of short, oblique, regular ribs proceed, which end about the 

 middle of the whorl in a rounded tubercle on the mould, and a thorny spine when the test 

 is preserved, from each tubercle a fasciculus of two or three striae passes across the area and 

 join those from the opposite side ; the siphonal area is convex and depressed, and well 

 defined by the oblique tubercles developed on the margin, from whence the fasciculi of 

 transverse striae proceed to unite with their fellows from the opposite side along the 

 middle line of the area. The whorl is widest along the row of the mucronated ribs, 

 whence it slopes sharply inwards towards the spiral suture, for this reason, the 

 umbilicus is deeply concave ; the line of the spiral suture lies outside the tubercles, 

 and the umbilicus thus acquires an ornate appearance. The aperture is transverse, 

 depressed, arched, and angular on the sides ; the septa are symmetrical, and the lobe- 



