ZONE OF HARPOCERAS BIFRONS. 135 



considerations have led me to regard the supposed varieties as distinct species, whatever 

 may be the affinities or the minghngs they have had in common in other countries.'' 



2. The zone of Harp, bifrons. " This zone is marly, and the change which has taken 

 place permits us to assign to it a precise lower limit. But it is not so easy to find the 

 upper boundary, and it is even necessary to make the palaeontological characters 

 intervene ; but we often meet at the base of the zone following with Li/to. jurense, a 

 bed of Tissoa — the compressed condition of which gives it the aspect of a true limestone. 

 It is this bed, the thickness of which is 10 — 15 centimetres, entirely enclosed in the 

 marls of the two zones, which appears to form the upper limit of the zone of Harp, 

 bifrons ; it is, in fact, above and below this bed that we observe a considerable change 

 in the two faunas. The marls are black and entirely argillaceous ; and enclose a large 

 number of species, of which the representatives are distributed according to fixed laws in 

 different levels of that zone." 



" a. At the base we find Step/ian. Braunianum. It is in this horizon that we discover 

 that fauna so curious and so new, and which is enclosed in a marly bed immediately in 

 contact with the Pos^Voram-shales. The marls in which this fauna is enclosed do not 

 exceed a metre and a half in thickness, and the principal species found here are : 



Stephanoceras Braunianum, tVOrb. 



— crassum, Phill. 



— Zitteli, Oppel. 

 Phylloceras subcarinatum. Sow. 

 Lytoceras Nilssoni, Hebert, 



— cornucopise, Soiv. 



Lytoceras Argelliezi, Hey. 

 Aegoceras acanthopse, d'Orb. 

 Harpoceras falciferum. Sow. 

 Nucula Paulae. 

 Cerithium hexagonum. 

 Rhyncbonella Julii. 



■ ' b. In the middle zone we recognise the true station of Harp, bifrons, and the fossils 

 are very numerous. 



Belemnites irregularis, SeJdoth. 



— acuarius, Schloth. 



— tripartitus, Schloth. 

 Nautilus semistriatus, d'Orb. 

 Harpoceras falciferum, Sow. 



— bicarinatum, Munst. 



— bifrons, Brug. 

 Lytoceras Nilssoni, Hebert. 

 Stepbanoceras crassum, Phil. 



— subarmatum, Young. 



— Emilianum, Mey. 



Aegoceras Gervaisi, Rey. 

 Harpoceras Erbaense, Hauer. 

 Plicatula Neptuni. 

 Lima gigantea. 

 — pectinoides. 

 Pecten incrustatus. 

 Area Bixa. 

 Nucula ovum. 

 — Delila. 

 Avicula Delia. 



" c. The upper part of this zone no longer includes Harjjoceras bifrons, and we only 

 meet in this horizon Mviih Z?/foceras Nilssoni, Harp, bicarinatum, Harpoceras falciferum, 

 Harp. JErbaense, Stephanoceras subarmatum, and Phylloceras heterojjhyllum. 



" This remarkable distribution of the species, according to distinct horizons, is almost 

 general, with rare exceptions, and we are able to assign beforehand to each species the 



