134 



THE LIAS AMMONITES. 



horizon with Ilellispongia fasciculata. The fourth zone, composed of compact marls 

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



In the Isere at Verpilliere and St. Quentin the Posidonomi/a-heAs are formed of 

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

 relation of the Posidonomya to the Jurense-7,one is likewise very well displayed. 

 The following species are characteristic of the Fosidonomya-heds at Verpilliere : 



Belemnites acuarius, Schlijih. 

 Harpoceras bifrons, Brug. 

 Lytoceras cornucopise, Toung Sf Bird. 

 Stephanoceras anmilatum. 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 

 Fosidonomya- and J«fe«se-beds 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 

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

 parts: — 1. The zone oi Harp, serpentimim ; 2, the zone oi Harp, hifrons ; 3, the zone 

 of Lyto.jiirense ; and 4, the zone of Harp, opalinum. 



1, The schists with Posidonia Bronni are from 3 to 30 metres in thickness; they 

 split into thin laminee, on which are impressed the marks of fossils, as Aptyclms 

 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, fossiliferous nodules, which 

 contain uncrushed shells in good preservation. 0]i breaking up the nodules the 

 following species were collected : 



Harpoceras serpentimim, Bein. 



— Levisoni, Simp. 

 Stephanoceras commune, Sow. 



Aptychus Lythensis. 

 Posidonia Bronni, Voltz. 

 Anodonta Bollensis, (iuenst. 



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

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

 as the prototype of Harp, hifrons. 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, hifrons. These diverse 



'Essai do Geologie et de Paleontologie Aveyronnaises,' p. 63, If 



