286 



THE LIAS AMMONITES. 



branches. A small accessory lobe lies between it and the siphonal lobe, which is 

 exaggerated in size in the drawing. The infero-lateral is small, and leads down to a 

 small umbilical or auxiliary lobe. 



Dimensions. — The following are the dimensions of four shells of from three to six 

 whorls in each, with their parts and number of ribs, from some typical specimens 

 before me : 



Number of whorls 



3 



4 



5 



6 



Do. ribs 



. 



22 



25 



36 



Diameter of shell 



15^ mm. 



33 mm. 



58 mm. 



67 mm 



Width of umbilicus . 



71 



' 2 5» 



17 „ 



31 „ 



34 „ 



Height of the keel at aperture . 



. 



11 



1 4 >> 



2 1 



3 „ 



Do. last whorl 



5 „ 



10 „ 



16i„ 



19 „ 



Do. penultimate whorl 



3 „ 



6 „ 



Ql 

 •^2 » 



11 „ 



Thickness of last whorl . 



41 



^2 5» 



8 „ 



11 „ 



12 „ 



Do. penultimate . 



91 



^2 >> 



^1 



°2 » 



73 



• 4 >» 



8 „ 



Young and Bird's figure of this shell is both bad and deceptive, and their description 

 incomplete ; hence arose early confusion about the species which they were the first to 

 name, although it was well known to the Yorkshire collectors, who greatly prized the 

 slabs crowded with its shells. Roemer collected it in Brunswick, and described it 

 accurately, but erroneously identified it as Am. natrix, and placed it among the 

 Capricorni. Quenstedt, in his f Jura,' has figured and described it under the name 

 falcaries, and Wagener under the names nodosaries and spinaries. In the Berlin Museum 

 some English specimens, obtained from a dealer, have the true name Am. semicostatus. 

 In the Jardin des Plantes the d'Orbignian collection contains the specimens which were 

 figured in the ' Paleontologie Erancaise' as Am. kridion, which I have examined. These 

 shells have been collected from several well known Lower Lias localities, as Villefranche, 

 Saone-et-Loire ; environs of Lyons, Rhone ; and Semur, Cote-d'Or ; Avallon, Yonne. An 

 Ammonite was collected from the Lower Lias Limestone near Stuttgart, and called 

 Kridion by Bergrath Hehl, and under this name it was figured by Zieten in his large work 

 on Wurttemberg fossils. This shell has been frequently mistaken for A. semicostatus, but 

 appears to be distinct ; it was erroneously identified by d'Orbigny with the French forms 

 of A. semicostatus, and hence our species appears as A. kridion in the ' Paleontologie 

 Francaise/ 



Dr. Oppel, overlooking the fact that Young had figured, and Simpson well described, 

 A. semicostatus, considered the shells he found in Robin Hood's Bay and at Lyme 

 Regis a new species, and described them under the name A. geometricus, the A. geome- 

 tricus, Phillips, having been found to be a form of A. spinatus. 



Locality and Stratigraphical Position. — This Ammonite is found in the Turneri-beds 

 or upper portion of the zone of Arietites BucMandi at Broad Ledge, Lyme Regis, 



