316 THE LIAS AMMONITES. 



Ammonites liassicus, d'Orbigny. Pal. fran?., T. Jurass., t. i, p. 199, pi. xlviii. 

 — tortilis, — Ibid., t. i, p. 201, pi. xlix. 



Diagnosis. — Shell discoidal, depressed ; whorls from six to eight, vertically flattened 

 or rounded and slightly involute ; umbilicus wide, and spire fully exposed ; the convex 

 sides ornamented with from forty to fifty prominent oblique ribs all directed towards the 

 aperture, larger near the umbilical, and diminishing towards the outer margin ; siphonal 

 area broad, sometimes marked with slight elevations of obsolete ribs. Lobes and saddles 

 well developed, lobe-line very sinuous. 



Dimensions : large specimen (figured PI. XVI) — transverse diameter 175 millimetres ; 

 width of the umbilicus 120 millimetres; height of the aperture 30 millimetres; width of 

 ditto 36 millimetres. 



Smaller specimen (figured PL XV, figs. 10 — 12) — transverse diameter 100 milli- 

 metres; width of the umbilicus 68 millimetres; height of aperture 16 millimetres; 

 width of ditto 20 millimetres. 



Description. — This interesting Ammonite belongs to the group Psilonoti in the zone of 

 Jngulatum, from whence I collected several fragments in the Harbury cutting of the 

 Great Western Railway, near Southam, Warwickshire, and which I found to be nearly 

 identical with a specimen sent to me by Monsieur Etallon, from the Lower Lias of France, 

 as a type of A. tortilis, d'Orbigny. Some time afterwards I obtained the large specimen 

 figured in PI. XVI, and which resembled some specimens, about the same size, I saw in 

 the d'Orbigny Collection at the Jardin des Plantes, Paris, and labelled^. Liassicus. As 

 neither of my determinations were quite satisfactory, I have constantly been in quest of 

 other figures, and now I find that all my specimens agree very closely with the Ammonites 

 laqueolus, Schlonbach, carefully figured in his valuable ' Beitrage zur Palaontologie der 

 Jura und Kreide-Formation in Nordwestlichen Deutschland,' from a specimen collected 

 by Herr Beckmann from the Angulatum-heds of Salzdahlum. In consequence of this 

 species being so extremely evolute, it is almost always found in fragments ; one of these I 

 have figured (PI. XV, fig. 1) from a specimen collected in the Angulatum-zone in the 

 Trent Valley, Lincolnshire, by the Rev. J. E. Cross, F.G.S. ; and I have seen fragments in 

 the British Museum, collected near Horfield by Bristol, where it is called A. Johnstoni. 



This Ammonite (PI. XV, fig. 10) is extremely discoidal ; the whorls are numerous, 

 very slightly involute, and well exposed. All the specimens I have seen want the centre, 

 so that the number of whorls is uncertain ; in Professor Schlonbach's figure of a 

 complete specimen there are seven. They are well rounded, rather convex on the sides, 

 and a little depressed at both margins (figs. 2 and 10), and in some specimens very much 

 so, as in the Harbury specimen (fig. 2) and in the large specimen from Redcar 

 (PI. XVI). The convex sides of the whorls are ornamented with from fifty-five to sixty 

 ribs, which are sharp and prominent, and all bend obliquely forwards towards the aperture ; 

 they are larger and more prominent near the umbilical margin (PI. XV, figs. 2 — 10), and 

 become smaller near the outer margin, where they disappear. 



