160 
IV. AcGocEeRatipz”. 
19. Aigoceras, Waagen. 
20. Arietites, Waag. 
21. Harpoceras, Waag. 
22. Oppelia, Waag. 32. Hoplites, Newm. 
23. Haploceras, Zittel. 33. Acanthoceras, Newm. 
| 29. Perisplinctes, Waag. 
24, Stephanoceras. Waag. | 34. Stoliczkaia, Newm. 
30. Olcostephanus, Newm. 
31. Scaphites, Park. 
25. Cosmoceras, Waag. 35. Crioceras, Léveillé. 
26. Ancyloceras, D’ Orbigny. 36. Heteroceras, D’ Orb. 
27. Baculina, D’Orb. 37. Peltoceras, Waag. 
28. Simoceras, Zitt. 38. Aspidoceras, Zitt. 
The first family, Arcestide, includes all the remarkable forms 
found so abundantly in the Triassic strata of the eastern Alps, 
and especially in the Red Alpine Limestone of that age at 
Hallstadt. The singular form of the lobe-line in all the 
species of this family is very different from the lobe-line of the 
Ammonites of the Jurassic and Cretaceous rocks. 
The second family, Tropidide, have the shell more or less 
ornamented and provided with radial ribs, which almost always 
support tubercles or spiny processes on the border of the 
siphonal area. Like the former family all the species are 
Triassic. 
The third family, Lytoceratide, includes several groups which 
differ from each other much as regards their external form, 
but which nevertheless appear to have close genetic relations 
in their internal structure; they all have a short body-chamber, 
about two-thirds of a whorl in length, a simple mouth-border, 
slightly produced on the columellar side, but in all the other 
features there is a wide differentiation of structure. This 
family includes the typical forms of Lytoceras and Phylloceras, 
and others which differ much from them in external form; 
as Hamites, Turrilites, and Baculites. 
The fourth family, Mgoceratide, includes a considerable 
number of groups which are widely different in form and 
affinities, so that itis difficult to give a general description of 
the family which shall be correct and comprehensive at the 
same time. We divide them, therefore, into three sections, 
