134 



THE LIAS AMMONITES. 



horizon with Hellispongia fascicidata. The fourth zone, composed of compact marls 

 with few fossils, is characterised by the presence of Steph. mucronatum. 



In the Isere at Verpilliere and St. Quentin the Fosidonom?/a-heds are formed of 

 ferruginous layers with very fine fossil shells in a high state of preservation ; and the 

 relation of the Fosidonomya to the Jurense-zane, is likewise very well displayed. 

 The following species are characteristic of the Fosidono?ni/a-heds at Verpilliere : 



Belemnites acuarius, Schloth. 

 Harpoceras bifrons, Sruff. 

 Lytoceras comucopise, Young ^ Bird. 

 Stephanoceras annulatum. Sow. 

 — commune. Sow. 



Stephanoceras Holandrei, d'Orb, 



— crassum, Phil. 



— fibulatum, Sow. 



— subarmatum, Sow. 



— anguinum, Rein. 



In the Loere and the Aveyron the Upper Lias is well developed, and both the 

 Posidonomya- and Jiirense-\iQd'S, are here found in natural superposition. The late 

 Dr. Reynes,^ after carefully studying these beds in the Aveyronnaise, divided them into 

 four zones, and observed that the faunas have very little affinity with each other, — " we 

 only ascertain but very rarely the appearance of a species beyond the zone in which it 

 has taken its entire development, and it is still more rare that we see isolated examples 

 ever appearing in beds of a later horizon." To facilitate the stratigraphical and 

 palaeontological study of the Upper Lias this author divided the Upper Lias into four 

 parts: — 1. The zone oi Harp, serpentiniim ; 2, the zone oi Harp, bifrons; 3, the zone 

 of Lyto. jurense ; and 4, the zone of Harp, opalimim. 



1. The schists with Posidojiia Bronni are from 3 to 30 metres in thickness; they 

 split into thin laminae, on which are impressed the marks of fossils, as Aptychus 

 Lythensis, Harp, serpentinum, Posidonia Bronni, and others. The shells are so much 

 compressed that it is difficult to restore their former character ; fortunately, however, 

 these laminated shales contain many large, hard, calcareous, fossihferous nodules, which 

 tontain uncrushed shells in good preservation. On breaking up the nodules the 

 following species were collected : 



Harpoceras serpentinum, Rein. 



— Levisoni, Simp. 

 Stephanoceras commune, Sow. 



Aptychus Lythensis. 

 Posidonia Bronni, Voltz. 

 Anodonta BoUensis, Quenst. 



" The fauna is not numerous, and none of the species of this hst ascend into the zone 

 of Harp, bifrons. It is true that some palaeontologists have considered Harp. Levisoni 

 as the prototype of Harp, bifrons. In this case they are under the necessity of 

 recognising two distinct varieties, the one having a lateral groove, and the other deprived 

 of that canal ; they are also forced to admit that these two varieties have lived at two 

 distinct epochs and without any intermixture, and that this modification is made in a 

 sudden manner with the appearance of the fauna of Harp, bifrons. These diverse 



^ 'Essai de Geologic et de Paleontologie Aveyronnaises,' p. 63, 18G8. 



