ORDER IL AMMONOIDEA 591 
Permian and Triassic genera here associated in the same sub-order are parallel with 
normal Triassic Ceratitoids, and have a wide distribution in the Trias. The Glosso- 
campyli is an aberrant stock with peculiar sutures. They begin in the Devonian, but 
have not yet been traced with certainty in the Carboniferous. They reappear, how- 
ever, in the Permian, and are represented by a single family in the Trias of India and 
Northern Siberia. The Devonian and Permian genera are widely distributed. 
(3) Ceratitoids—The sub-order Discocampyli is restricted to the Trias with the 
exception of a single family, Flemingitidae, which occurs in the Permian of India 
and North America. The forms have a world-wide distribution, and are especially 
numerous in India and the western United States. This group attains the acme of 
specialisation among Ammonoids, as shown by their extremely complex sutures with 
numerous lobes and saddles, combined with a highly ornamented exterior and con- 
siderable modification of form. The number of retrogressive or phylogerontic genera 
is also very small. 
(4) Ammonitoids.—The Phyllocampyli include the supposed ancestral and primitive 
forms of typical Ammonoids, and begin with the family Prolecanitidae, which ranges 
from the Devonian to the Permian. Three primitive offshoots of this family are 
present in the Permian, two of which persist also during the Trias. In all, nine 
families of Phyllocampyli occur in the Trias, only one of which (Phylloceratidae) 
continues throughout the Jura and Cretaceous. Most of the familes have a world- 
wide distribution in the several formations where they occur. The sub-order reaches a 
high degree of sutural complication, and this, together with the number of lobes and 
saddles in the Phylloceratidae, mark them as being among the most progressive of 
Ammionoids in respect to sutures. Their shells, however, are very simple in comparison 
with either the Discocampyli or Pachycampyli. 
The Leptocampyli are obviously a retrogressive series. They begin at the base of 
the Jura with forms having a restricted number of lobes and saddles, and terminate in 
the Upper Cretaceous with two families of uncoiled phylogerontic shells. The com- 
plexity of the sutural outlines is, however, constantly maintained, as is especially well 
illustrated by Lytoceratidae, where the arms of the antisiphonal are curiously pro- 
longed. Their distribution coincides with that of the Phyllocampyli, and they are 
quite abundant in certain Jurassic and Cretaceous localities. 
The Pachycampyli are usually considered as the typical and also most progressive 
forms of Ammonoids. This is true as regards the immense number and variety of 
modifications in form and ornament displayed by their shells and apertures; but the 
sutural inflections are on the average less complex both in outline and number of 
lobes and saddles than those of the Phyllocampyli, although both groups are evidently 
derived from the same stock. 
By the term progression, as used above, is meant “evolution of structures increasing 
by differentiated additions,” such as the addition of more complex inflections to either 
the dorsal or external sutures, increase in the amount of involution, or the introduc- 
tion of keels and channels on a primitively convex venter, ete. Retrogression signifies 
“evolution through the reduction of such progressive characters,’ whether taking 
place locally or generally in the organism or throughout the group. Retrogression, in 
this structural sense, does not imply actual reversion to ancestral conditions, except in 
so far as the disappearance of a part or organ necessarily produces a certain resemblance 
to their ancestors before the parts or organs in question were evolved. Examples will 
be found on comparing Baculites with, Bactrites, Mimoceras with Crioceras, ete. In 
this sense Ammonoids experienced a progressive evolution from the early Devonian 
until the Upper Trias, when the first signs of general retrogression are observed, and 
a few uncoiled and turriliticonic genera appear. 
Following their culmination in the Trias, Ammonoids display in the Jura a 
mixture of retrogressive with some progressive features. Part of their losses are 
regained by the evolution of a vast number of forms and modifications during this 
