324 



THE LIAS AMMONITES. 



Ammonites Chakmassei, Studer. Geologie der Schweitz, vol. ii, pp. 30, 34, 1853. 



— antiquatus, Simpson ? Fossils of the Yorkshire Lias, p. 36, 1855. 



— Chabmassei, Von Hauer. Cephalopoden Lias N.-O. Alpen, p. 49, pi. xiv, 



figs. 1—3, 1856. 



— — Dumortier. Depots Jurass. du Bassin du Rhone, II, p. 29, 



pi. xvii, figs. 1—4, 1867. 

 Aegoceras — Tate and Blake. Yorkshire Lias, p. 272, 1876. 



Diagnosis. — Shell discoidal, depressed ; whorls compressed, slightly convex, one half 

 involute, sides ornamented with numerous unequal ribs, variable as to number, in general 

 thirty, bifurcate about the middle of the whorl and terminating at the margin in a blunt 

 thickening. Siphonal area narrow, subcarinate, and furrowed in the middle, sides 

 carinated with the two rows of blunt terminal ribs ; aperture high, narrow, sagittate ; 

 umbilicus moderately open ; lobe-line very sinuous, forming six lobes and numerous 

 saddles ; number of ribs at umbilical margin thirty, at siphonal margin sixty. 



Dimensions. — Transverse diameter of figured specimen 170 millimetres; width of the 

 umbilicus 45 millimetres ; height of the last whorl at aperture 80 millimetres ; greatest 

 width of ditto 40 millimetres. 



Prof. d'Orbigny's type specimen measured : — Transverse diameter, 235 millimetres. 

 The relations to the diameter in an adult were — height of the last whorl ^$q ; involution 

 of the last whorl j%q ; width of the umbilicus t£q ; thickness of the last whorl y^-. In 

 a young shell the width of the last whorl y^ ; width of umbilicus y % ; thickness of the 

 last whorl -f^. 



Prof. Von Hauer has given the dimensions of a very fine specimen. The transverse 

 diameter was 6 inches = 1 54 millimetres ; the height of the last whorl y^j of the 

 diameter ; width of last whorl -f^ ; the width of the umbilicus -f^. 



M. Eug. Dumortier, ' Depots Jurassiques,' vol. ii, p. 29, has given the dimensions of 

 three fine specimens with their test, belonging to M. Ed. Pellat's collection, and all 

 obtained from the same quarry of Lower Lias at Drevain, Saone-et-Loire. 



Parts Measured. 



No. 1. 



No. 2. 



No. 3. 



Height of the last whorl in relation to the diameter 



Thickness of the last whorl 



Umbilicus, width of 



2(H millim. 



3 6 

 100" 



31 millim. 

 34 „ 

 44 „ 



76 millim. 



45 



25 „ 



23 „ 



52 „ 



225 millim. 



47 „ 



21 „ 



22 „ 

 78 



Number of ribs, outer margin 





Description. — The form and structure of this Ammonite were very variable in the 

 different phases of its growth. In youth, and when it has a diameter of eighty milli- 



