832 THE LIAS AMxMONITES. 



Third Section. — Armati. 



Aegoceras BiRCHii, Soicerhy. PI. XXIII; PI, XXXII, figs. 5—8. 



Ammonites Bikchi, Sowerby. Mineral Conchology, vol. iii, p. 121, pi. 267, 1820. 

 Planttes — Haan. Ammon. und Goniatit., p. 82, No. 2, 1825. 



Ammonites — Quenstedt. Petrefactenkunde ; Cephalopoden, p. 86, 1849. 



— — d'Orbigny. Pal. rran9aise ; Terr. Jurass., p. 287, pi. 86, 1842. 



— — Oppel. Die Juraformation, p. 84, 1856. 



Aegoceras — Neumayr. Zeitschrift der Deutsch. geol. Gesellschaft, B. xxvii, 

 p. 906, 1875. 



— BiEcnii, Tate and Blake. Yorkshire Lias, p. 274, 1876. 



Diagnosis. — Shell discoidal, compressed ; whorls eight, narrow, sub-rotund, and 

 slightly involute ; sides compressed with numerous thick, blunt, straight ribs ; on each 

 of these two sharp tubercles are developed, which are all visible in the wide, open 

 umbilicus ; the tubercles are very sharp in young, and blunt in old shells ; aperture sub- 

 quadrate, with lateral points only slightly grooved by the turn of the spine. 



Dimensions — Large specimen. — PI. XXIII. Diameter 160 millimetres ; width of 

 the umbilicus 95 millimetres ; height of the last whorl 35 millimetres ; width of ditto 35 

 millimetres. 



Small specimen. — PI. XXXII, figs. 5 — 8. Diameter 80 millimetres ; width of the 

 umbilicus 50 milUmetres ; height of last whorl 15 millimetres; width of ditto 15 

 millimetres. 



Description. — This very distinct and characteristic Ammonite of the Lower Lias is 

 readily distinguished by the round siphonal area with its numerous bi-tuberculated ribs. 



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

 inches in diameter, there are only seven volutions of the shell ; the sides of the whorls are 

 convex and slightly flattened in the middle, they have from thirty-two to thirty-four 

 thick, straight, lateral ribs, with two tubercles developed on each j the outer tubercle is 

 the largest, and in some specimens becomes spinous ; the inner is smaller and more 

 obtuse, and the ribs disappear near the inner border. 



The siphonal area (PI. XXIII, fig. 2) is round, and in some specimens three or four 

 fine transverse bands extend across the area from one outer tubercle to the other, and 

 fine transverse lines of growth are seen on the shell figured in PL XXIII. 



The spire is formed of very narrow whorls, which are slightly involute, and all are 

 fully exposed. 



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

 turn of the spire (PI. XXIII, fig. 2). 



The septa are symmetrical, and have an extremely complicated arrangement of lobes 

 and saddles ; of these I am only able to figure a portion (PL XXXII, fig. 7). The 



