534 MOLLUSCA SUB-KINGDOM VI 
yet been found in the Carboniferous, where uncoiling of the volutions, when it 
occurred, took place earlier than the gerontic stage. From the Mesozoie and later 
horizons, no species is known in which the gerontic stage is to the slightest degree 
uncoiled. 
Torticones are more aberrant than any other conchs, and may be best classified as 
phylogerontic forms, since tendencies toward unsymmetrical development of the 
volutions occur in the gerontic stage, and are genetic in but a few genera, where they 
appear during the early stages and are preceded so far as known by a symmetrical 
volution. The first manifestation of torticones is in the Ordovician, and their acme is 
attained during the Silurian. As regards ornamentation, annulated shells appear in 
the Calciferous, and those with longitudinal ridges later in the Ordovician, together 
with tuberculated and costated gyroceracones and nautilicones. The last-named, how- 
ever, are much more abundant in the Devonian and Carboniferous, after which they 
disappear. Very highly ornamented shells exist in the Trias, but following this 
period the conchs are smooth. 
Very striking is the marvellously sudden rise of the Nautiloidea as a group, 
reaching its maximum in the Silurian, and followed by a decline extending from the 
Devonian to the Trias. Then the forces acting unfavourably upon their existence 
were arrested, or their violence lessened, and the group has been affected by only very 
slight changes and an exceedingly slow process of retrogression until the present time. 
The acme of siphuncular differentiation occurred in the Ordovician, of general morphic 
diversity in the Silurian, of ornamentation in the Devonian, and of sutures in the 
Trias. 
Geographically considered, some facts of distribution are of general interest. The 
fauna of the Quebee or Calciferous, which in Newfoundland, Canada, Vermont, and 
the vicinity of Poughkeepsie, New York, is rich in fossil remains, is represented by 
but a few camerated conchs in the Durness Limestone of Scotland. Holochoanites and 
Schistochoanites are most plentifully represented in the American faunas, but Mizo- 
choanites very sparsely so, at least as compared with the Ordovician and Silurian of 
Bohemia. The same is true of the Lituitidae, Ophidioceratidae, and Hercoceratidae 
among Orthochoanites, and of the Jovellanidae, Trimeroceratidae, and kindred families 
among the Cyrtochoanites. The Devonian and Carboniferous faunas of America and 
Europe are nearly on a par, but the Permian and Trias of the western hemisphere are 
very deficient in Nautiloid remains. The Jurassic faunas of America have so far 
yielded but one specimen of a Nautiloid, but they were probably present to some 
extent, since they are represented in the Cretaceous of this country. During the 
Cretaceous and Tertiary the principal distribution of the Nwutilidae was in the 
eastern hemisphere, and the last surviving species of Nautilus are now restricted to 
oriental waters. The accompanying table shows the range of the leading Nautiloid 
families. 
(The Hercoceratidae occur in the Devonian, and not in the Silurian rocks, as is stated 
through an oversight on page 524. ] 
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