390 THE LIAS AMMONITES. 



The morphology of this Ammonite has been closely studied ; when examined at 7 

 millimetres in diameter, the sides of the whorls are slightly more convex in proportion 

 than in older shells, and ten or twelve wide folds are noticed aromid the umbilicus, the 

 first indications of the lateral ribs ; at 20 millimetres, the shell remains still proportion- 

 ally thicker, the carina very little elevated forms a small round keel separated from the 

 ribs, and the shell in this condition resembles Am. maeandrus, Reinecke; at 50 milli- 

 metres, the shell assumes its typical form, which it retains to 130 millimetres; it now, 

 however, begins to lose its ribbed foldings on the body whorl; and at 160 millimetres, 

 the cutting edge becomes thickened and the shell less compressed ; but from first to last 

 the specific characters scheduled in the diagnosis are well preserved throughout its life 

 history. 



The study of some large specimens of this Ammonite with their shell well preserved, 

 enabled the late M. Duraortier^ to complete an observation he had already made on other 

 Ammonites, on the structure of the shell in a good state of preservation. He found the 

 test to consist of two very distinct laminae ; an internal layer very thick near the umbilicus, 

 diminishing progressively in proportion as it approaches the margin, where it is as thin 

 as a sheet of stout paper; and an external layer, very thin on the contrary, near the 

 umbilicus, and thickening progressively as it approaches the margin, where it acquires a 

 thickness of S millimetres. By this arrangement a process of compensation is established 

 by which the general thickness of the shell is maintained throughout. 



Affinities and Differences. — Amaltheus oxynotus resembles three other species asso- 

 ciated with it in the same zone, these are Amal. Lymensis, Amal. Simpsoni, and Amal. JFilt- 

 sJiirei. It differs from Am. Lymensis in having an open umbilicus, a lesser convexity of 

 the sides, and a different style of the ribs ; from Amal. Simpsoni in having a more com- 

 pressed shell, much flatter around the umbilicus, and a different texture of shell 

 structure ; from Amal. Wiltsldrei, with its closed umbilicus, in possessing an open umbi- 

 licus ; ribs entirely different, and a distinct shell structure. 



Locality and StratiyrapJiical Position. — This Ammonite is the leading fossil in beds 

 which separate the zone of Arietites obfusus from the zone of Arietites raricostatus, and 

 which are so well shown in Gloucestershire, Dorsetshire, and Yorkshire (see p. 52). 

 This bed is very rich in many species of Ammonites. 



Foreign Distribution. — It is found in Swabia and Wiirtemberg. In France it was 

 collected in abundance, according to Dumortier, at St. Fortunat, St. Cyr, Lournand, St. 

 Helene, and Jambles ; and, according to Oppel, he found very large specimens in the 

 Blue Limestone of Nancy. 



It is singular that d'Orbigny has not figured a true Amal. oxynotus in his Paleon- 

 tologie Fran^aise, although it certainly occurs in abundance in the basin of the Rhone. 

 Ammonites lynx, d'Orb., Ammonites Coynarti, d'Orb., and Ammonites Collenoti, d'Orb., 



1 'Depots Jurassiques du Bassin du Rhone,' ii, p. 145. 



