220 INFERIOR OOLITE AMMONITES. 



The inferior lateral lobe in d'Orbigny's drawing is proportionately larger than 

 in mine. 



Dr. Wright states that this species is the leading fossil of the Opalinum-zo'ne 

 [op. cit., p. 468). It is true that at a locality like Coaley Wood the species occurs 

 but a very little distance, about eight inches, below Lioc. opalinum. Yet, geo- 

 logically, the interval which separates the two species is very considerable, 

 though the deposit is small. One distinct Ammonite-horizon intervenes ; and 

 though only about eight inches thick at Coaley Wood, yet it is over 100 feet thick 

 in the Dorset-Somerset area. 



The correct position of this species is a matter of importance, because it is a 

 form so unmistakable. Its actual horizon is in strata which I have called 

 " Dispansiim-heds,^^ and which I have placed' as one of the middle divisions of the 

 Jurense-zone} It shares this horizon with many species, of Avhich three are very 

 characteristic, namely, Gh^amm. dispansum, Gramm. fallaciosum, and Hamm. 

 insigne. 



I have collected this rare species in Grloucestershire at Coaley Wood and North 

 Nibley ; in Somerset at White Lackington. I have also specimens from Milhau 

 (Aveyron), France, and from La Porcarella (Central Apennines), Italy. Meneghini 

 speaks of this species being common in Umbria and in the Toarcian of the 

 Apennines, but scarce in Lombardy. It is certainly one of those species which 

 have been regarded as having their head-quarters on the borders of the Medi- 

 terranean, and which seem to become gradually rarer further north. 



The figures of Pohjplectus discoides which I have given are merely to illustrate 

 the lobes and saddles and my description thereof. PI. XXXVII, fig. 1, is the 

 suture-line, of natural size, from a specimen obtained at White Lackington, Somerset. 

 Fig. 2 is the side view of a small specimen from Milhau, Aveyron (France). Fig. 3 

 is the front view of the same to show the lobes and saddles, and to demonstrate 

 that the omission, in d'Orbigny's drawing, of the accessory lobe, a a, of the 

 siphonal saddle was a mistake. Fig. 4 gives a portion of the terminal branch 

 of the siphonal lobe from the same specimen where the whorl is 3^ lines broad. 

 Fig. 5 illustrates the same part from the same specimen where the whorl is 5f 

 lines broad, showing how the lobe proceeds to develop the accessory tuft. 



^ See p. 166, foot-note 1. 



2 Haug (op. cit., p. 621) states that the species occurs with Lytoceras jurense. 



