463 THE LIAS AMMONITES. 



accessory lobe. The principal lateral lobe, large and oblong, is adorned on each side 

 with five ramified branches, not including the terminal branch. The lateral saddle, 

 nearly as large as the principal lateral lobe, is surrounded by nine folioles. The 

 auxiliary lobes are seven in number, the first is large and the inner six rapidly diminish 

 in size as they approach the umbilicus and are separated by as many small saddles with 

 foliated walls. A radial line extended between the siphonal lobe and the umbilicus 

 would pass beneath all the other lobes. 



Prof. d'Orbigny 1 had the opportunity of examining a young form of Harp, discoides, 

 he says at the diameter of 5 millimetres it has a smooth, round, siphonal area, which soon 

 becomes angular but remains smooth up to a diameter of 10 or 11 millimetres. The 

 simple transverse costae and sulci afterwards begin to appear, and are nearly as equally 

 spaced out as they are in the adult. Each additional whorl contains a greater number 

 than the penultimate whorl, and thus the number of the costae and furrows increases 

 with the growth of the shell. 



Affinities and Differences. — This species very much resembles Harp, exaratum in 

 the width of the whorl, and in the flexures of the costae, which in both are simple and 

 distinct throughout, both have a small umbilicus with a similar involution of the 

 whorls. In the siphonal area lies the difference, Harp, discoides has a sharp trenchant 

 area, whereas in Harp, exaratum there is a central keel with two lateral furrows, and 

 two outer carinae bounding the area. 



Locality and Stratigrapldcal Position. — I have collected three or four specimens of 

 this shell from the Upper Sands at Erocester Hill in the zone of Harp, opalinum. The 

 largest example I possess is the one now figured. In France it has been obtained from 

 many localities where the Harp .-opalinum zone crops out as at Milhaud, Aveyron ; near 

 Fontenay, Vendee ; Sant-Rambert, Ain ; Uhrviller, Bas Rhin ; near Lyons, near Salins, 

 Jura ; la Verpilliere, Isere ; Privas, Ardeche ; Eressac, Gard ; la Cride, Var ; St. Fortunat, 

 Rhone ; St. Romain, Rhone ; whenever this beautiful little Ammonite is found it may be 

 considered to be a leading fossil of the Harp .-opalinum zone. 



1 ' Paleontologie Francaise, Terrains Jnrassiqucs,' torn, i, p. 357. 



