290 THE LIAS AMMONITES. 



Dimensions. — Transverse diameter of specimen figured on Plate XIII, 150 milli- 

 metres; height of the last whorl, 50 millimetres; thickness of ditto, 35 millimetres; 

 amount of involution, 8 millimetres ; diameter of the umbilicus 60 millimetres. 



Description. — This is a very remarkable Ammonite. In early life, its specific 

 characters are well developed ; in middle age it changes much ; after attaining a very large 

 size in its adult state it differs from all the previous conditions, as shown in specimens 

 contained in the Museum at Semur, where this species may be studied in the different 

 states of development. The shell is very thin, much compressed and carinate, provided 

 with an elevated, but not cutting keel. The sides are flat and ornamented with numerous 

 ribs, which vary in number in different individuals (from eighteen to thirty-six). They 

 are thick and well developed when they appear at the umbilicus, and rise in straight lines, 

 gently bent forward towards the aperture, and near the margin of the siphonal area 

 terminate in small blunt tubercles. In the specimen I have figured from the Lower Lias 

 of Gloucestershire (PL XIX, figs. 8 — 10) the ribs (seven in number) are far apart, the 

 tubercles are small and conical, and the keel is sharp and prominent. In another 

 specimen, about the same size as the latter, there are twenty-six ribs, so that the 

 number of these lateral folds is valueless as a specific character in this form. In a larger 

 specimen from the Museum of Semur, 165 millimetres in diameter, there are thirty-six 

 ribs ; and the fine specimen I have figured (PI. XIII) from the same locality has twenty- 

 eight ribs. The admirable figure given in this plate shows likewise another important 

 feature in this Ammonite, namely, that the ribs, tubercles, and keel are much sharper 

 and better defined in the cast than in specimens covered with the original shell. 

 A small Semur specimen in my hand shows this very distinctly, and is verified, in PI. 

 XIII, in that portion of the outer whorl which has retained its shell. The ribs likewise in 

 this species are rarely bifurcated. 



The terminal costal tubercles appear to be best developed in early age ; they gradually 

 become blunted in the middle period, and finally disappear in the senile condition. 



The keel is as well developed upon the cast as in those parts covered with the 

 shell, and has almost always the same prominence ; an anatomical fact of some importance, 

 seeing that in a congeneric form, the A. Abattoensis, from a higher zone, as pointed out 

 by Mons. Dumortier, the shell possesses a well-developed keel, which is entirely absent in 

 the cast. 



The lobe-line is delineated on Plate XIII ; a lateral view in fig 1, and the siphonal 

 and lateral lobes and saddles in fig. 3 ; this outline is essentially the characteristic form 

 of the group Arietites to which this species belongs. 



The aperture is compressed on the sides and forms an irregular hexagon ; the two 

 siphonal facets form prominent angles, the two sutural obtuse angles, and the lateral 

 facets are inclined outwards. 



Since the above description was in type, I have received, through the extreme kind- 

 ness of M. Collenot, of Semur, a larger specimen of this species than the one figured 



