168 



First Group, Arietes, von Buck, 1829. 

 Ammonites with the shell in nearly all ornamented on the sides with 

 simple radiating and projecting ribs; the back rounded, angular, or square, 

 nearly always provided with a median keel, and a gi-oove on each 

 side thereof. Siphuncle ia the keeled forms prominent, situated in the 

 dorsal prominence. Mouth prolonged into a projecting process. Septa 

 formed of lobes and saddles, with unequal ramifications. The dorsal 

 lobe as deep as it is wide, and longer than the superior lateral lobes. 

 The lateral saddle extending in general further forward than the others ; 

 the dorsal saddle veiy short. This group contains forms which are 

 found only in the Lower Lias ; the species are distributed, through the 

 zones of Ammonites planorhls, angulatus, BucMandi, Tumeri, and 

 ohtusus, which they all well characterize. 



The group Arietes is divisible into two sections. 

 (A.) — Arietes without a keel, including three tvell-marked species. 

 Ammonites planorbis, Soiverhy. 



II ANGULATUS, Schlotheim. 



M Sauzeanus, d'Orhigny. 



(B.) — Arietes ivith a keel. 

 Ammonites Bucklandi, Sow. Ammonites obtusus, Sow. 



II CoNYBEARi, Sow. » BoNNARDii, d'Orbignij. 



II Turneri, Sow. h Brookii, Sowerby. 



II rotiformis, Sow. n semicostatus, Young & Bird. 



II stellaris, Sow. m bisulcatus, Brugui^re. 



The Arietes without a Keel. 

 Ammonites planorbis, Sow. (PI. i.. Fig. 1.) 

 Ammonites planorbis, Sowerby, Mineral Conchology, p. 448, 1824. 



M ERUGATUS, Young & Bird, Geol. of Yorkshire, tab. 13, 



fig. 13, 1829. 

 II psiLONOTUS, Quenstedt,FlbzgehixgeWm'iembergs,Tp. 127, 1843. 



n PSILONOTUS LjEVIs, Quenstedt., Cephalopoden, p. 73, tab. 3, 



fig. 18, 1849. 

 II Hagenowi, Dunker, Palaeontograpliica, p. 115, tab. xiii., fig. 



tab. xvii., fig. 2, 1846. 

 II planorbis, Oppel, Juraformation, p. 73, 18.56. 



II PSILONOTUS, Quenstedt, Handbuch der Petrefactenkunde, 



p. 354, 1852. 

 II PSILONOTUS L^vis, Quenstedt, der Jura, p. 40, 1858. 



M . Macdonellii, Portlock, Report on Londonderry, p. 134, 



PI. xxix. A., fig. 12, 1843. 



