534 MOLLUSC A 



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 Mesozoic 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 iiautilicones. 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 Quebec 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 Mixo- 

 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 Nautilidae 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.] 



