169 



Shell smooth, dlscoidal, compressed; back rounded without keel, 

 whorls four-fifths exposed, only slightly involute; surface of the thin 

 test covered with fine hair-like lines of growth. 



Dimensions.— Greatest diameter, 2 inches ; height of the last whorl at 

 mouth, seven-tenths of an inch ; mdth, half an inch ; whorls one-fifth 



involute. 



Description.— This Ammonite Ls found nearly always ci-ushed between 

 lamina of shale ; its natural fonn is therefore rarely seen, except in 

 some Yorkshire specimens. It has a smooth discoidal shell, covered 

 with fine, hail-like Unes of growth, (pi. 1, fig. 1,) the back is round, and 

 without a keel ; the whorls, six in number, increase rapidly in diameter, 

 and are four-fifths exposed, so that the whorls are only slightly involute. 

 Dorsal lobe small, and little divided ; superior lateral lobes long, five 

 digitations ; inferior lateral lobe shorter, three digitations ; ventral lobes 

 small, short, and few digitations. (Fig. 1, a, b.) 



This species includes two well marked varieties, (a) IxBvis, the form 

 which I have figured and described, and (6) pUcatws, which has ribs on 

 the lateral parts of the whorls. This variety has been figured by 

 SowERBY, in the Mineral Conchology, as Ammonites Johnstonii; and by 

 d'ORBiGNY, in the Paleontologie Frangaise, as A. tortilis, (tab. 49,) and 

 A. toi-us, (tab. 53.). It is described by Quenstedt, in Ms monograph 

 on the Cephalopoda, as A. psilonotus plicatus. 



A. Johnstonii has thirty short rounded ribs on the lateral sides of the 

 whorls, that do not extend across the back, nor to the sutui-e of the 

 enveloped whori. All the ribs completely disappear before they reach 

 the back, which is smooth, rounded, and without the trace of a keel. 



Affinities and Differences.— The character of the lobes connects this 

 Ammonite with the group Arietes to which it is refen-ed, but it wants the 

 keel, doi-sal furrows, and ribs, which distinguish most of the species of 

 that'gi-oup. The smooth forms exhibit sometimes folds on their sides, 

 and these pass by insensible gradations into the ribbed varieties, which 

 SowERBY figured as A. Johnstonii, and d'ORBiGNY as A. torus; the absence 

 of a keel, and the simplicity of the lobes, impart a peculiar character to 

 this fii-st Ammonite of the Lower Lias. 



Locality and Stratigraphical Position.— I have collected A. planmJm in 

 the first or lowest zone of the Lower Lias ; wherever these beds are exposed 

 it forms their most characteristic fossil. It is very abundant in the 

 shales of this zone, at Brockeridge and Defford Commons, and at Binton, 

 Wilmcote, and Grafton, Warwickshire ; at Street and Upliill, Somerset ; 

 Penarth, Glamorgan ; and at Pinhay Bay, and Uphill, near Lyme Regis, 

 Doreet; where it is found in the upper part of the Ught-coloured 



