AEGOCERAS JAMESONI. 353 



Ammonites Jamesoni, Wright. Quart. Joiirn. Geol. Soc, vol. xiv, p. 27, 185S. 



— — Chapuis. Terr. Second. Luxembourg, p. 32, tab. vi, fig. 1, 



I808, 



— — Schlvnbach. Eisenstein d. Mittl. Lias; Zeitsch. d. Deutsch. 



geol. Gesellscb., p. 523, Jalir 1863. 



— — Dumortier. Depots Jurass. du Bassin du Rlione, vol. iii, p. 93, 



1869. 



— — Emerson. Lias von Markoldendorf, Zeit. Deut. geol. Gesell., 



p. 274, 1870. 

 ..^GOCERAS — Tate and Blake. Yorkshire Lias, p. 279, 1876. 



Diapiosis. — Shell discoidal, more or less compressed ; whorls one thivd involute, 

 slightly depressed, and convex, ornamented with fifty-six slender, equal, simple ribs, with 

 sigmoidal curve, often developing a tubercle near the margin, and forming an arch across 

 the siphonal area and joining the ribs of the opposite side. Siphonal area narrow, convex, 

 and depressed; aperture oblong, compressed, and elongated. 



Dimensions. — Large figured specimen (PL LI, fig. 1) IGO miUimetres ; width of 

 umbilicus 75 millimetres ; height of aperture 50 millimetres ; width 37 millimetres. 

 Small Pabba fossil (PI. LI, fig. 4) — transverse diameter 76 millimetres ; width of 

 umbilicus 37 millimetres. 



Seldom found in a satisfactory condition in middle age. 



Description. — This Ammonite, very rare in England and Scotland, is much more 

 abundant in Wiirtemberg, whence my best specimens have been obtained (PI. XI, fig. 4). 

 Two very distinct varieties have there been described and figured by Professor Quenstedt ; 

 the first named — a, Aiu. Jamesoni latm, with thick ribs and a broad siphonal area, the 

 normal form of Sowerby ; the second — b. Am. Jamesoni angustatus, with more numerous 

 and slender ribs, a narrow, convex, siphonal area, and the aperture compressed and 

 elongated. 



A good type shell of Aeg. Jamesoni latum is figured in PI. LI, Iigs. 1 and 2, and a 

 good type of Jamesoni, angustatum in PI. LI, fig. 4. 



These figures show that there is a very considerable variation in the form and ribbing 

 of this elegant Ammonite. Sowerby 's type was a mere fragment obtained from the Lias 

 of the Island of Mull ; two of the specimens I possess were collected from the Middle 

 Lias of the Island of Pabba near the Island of Skye, and one of these is figured in PI. 

 LI, fig. 4. In the Hebridean fossils we have the true Sowerbyan type ; the shell is com- 

 pressed in early and middle age, the sides are ornamented with simple equal-sized ribs, 

 gradually increasing in length and thickness with the growth of the shell ; they arise at a 

 short distance from the spiral suture, and describe a slight sigmoidal flexure, the curve 

 being inclined towards the aperture ; near the margin the rib becomes thicker and forms 

 an arch across the siphonal area, which in old large shells is thick, wide, and depressed, 

 whilst in middle age it becomes convex, narrow, and prominent. In some specimens 

 the thickened marginal portion of the rib develops a blunt tubercle, and a feeble carina 



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