386 THE LIAS AMMONITES. 



Ammonites Guibalianus, Bronn. Gesch. d. Natur., iii, pt. i, p. 44. 



— — Oppel. Die Juraformation, p. 86, 1856. 



— — Bumortier. Depots Jurass. du Bassin du Rhone, ii, p. 140, 



1867. 



Diagnosis. — Shell compressed, carinated ; whorls compressed, and extremely involute ; 

 sides convex, numerous sigmoidal ribs extend from the spiral suture to the carina, with 

 short supplementary ribs on the upper half of the wider valleys ; siphonal area narrow, 

 with sloping sides, a central carina, and oblique transverse ribs, aperture elhptico- 

 lanceolate, much indented by the penultimate whorl ; lobe-line extremely complicated. 



Dimensions. — Transverse diameter of figs. G and 7, 133 millimetres ; width of umbilicus 

 20 millimetres; height of aperture 75 millimetres; width 35 millimetres. Figs. 3, 4, 

 transverse diameter 70 millimetres ; width of umbilicus 16 millimetres; height of aperture 

 40 millimetres, width 2:2 millimetres. Figs. 1, 2, transverse diameter 50 millimetres; 

 width of umbilicus 12 millimetres; height of aperture 27 millimetres; width? 



Description. — I have figured three specimens of this Ammonite and given their 

 respective dimensions to show that very little change takes place in the growth of this 

 species, which attains dimensions very much greater than the shell figured in 6 and 7 

 selected for the type figure of this fossil, because in early life its ribs are more defined 

 and in old age they become obsolete. M. Dumortier states " that his largest specimen 

 came from Lournand ; it is 245 millimetres in diameter; the width of the last whorl 

 46/100; the thickness 26/100; the size of umbilicus 18/100. The last whorl has 

 thirty-six ribs or folds slightly marked, and which appear less flexed than the ribs of 

 the inner whorls. This specimen is furnished with lobes up to its extremity, consequently 

 its transverse diameter when provided with its body-chamber must have exceeded 360 

 millimetres." My largest specimen measures 245 millimetres in diameter, the width is 70 

 millimetres, the height 120 millimetres, and the width of the umbilicus is 35 millimetres. 

 The shell is compressed and sharply carinated, and ornamented with from thirty-six 

 to forty ribs ; the number, however, differs in diff'erent specimens. Most of the ribs pass 

 from the spiral suture to the carina undivided, many of them, however, bifurcate, and a 

 number of short secondary ribs make their appearance between the primaries in the wider 

 valleys (fig. 7). In some young shells the ribbing is very regular (figs. 1 and 2), whilst in 

 others it becomes irregular as in fig. 4, in which the primary ribs are few in number and 

 the secondaries numerous. The ribs describe a sigmoidal flexure and bend forward 

 towards the carina, their increase in number depends either upon the bifurcation of the 

 primary rib or on the introduction of short small secondaries in the upper part of the whorl. 

 The spire is composed of six whorls which are extremely involute, and the umbilicus 

 is consequently deep and narrow, the inner whorls being mostly concealed by the matrix. 

 The siphonal area is narrow, and slopes down to the sides ; the keel varies, sometimes 

 it is obtuse, sometimes acute, and in general it is marked with fine lines of shell growth, 

 which impart a serrated structure to the carina when the shell happens to be preserved. 



