CLAss V CEPHALOPODA 615 



r 



appear during tlie early stages and are preceded so far as known by a symmetrical 

 volutioii. The first manifestation of torticones is in the Ordovician, and tlieir acme is 

 attained during tlie Silurian. As regards ornanientation, annnlated shells appear in 

 the Calciferous, and those with longitudinal ridges later in the Ordovician, together 

 with tiiberciilated and costated gyroceracones and nantilicone.s. The last-named, how- 

 ever, are nuich more abimdant in the Devonian and Carbon iferous, after which they 

 disappear. Very higlily ornaniented shells exist in the Trias, but following this 

 period the conchs are smootli. 



Very striking is the marvelloiisly sudden rise of the Nautiloidea as a gi'oup, 

 reaching its maximiim 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 

 sliglit 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 rieh in fossil remains, is represented by 

 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 Oi-dovician 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 of the western hemisphere is 

 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 following table shows the ränge of the leading Nautiloid 

 families. 



[The systeniatic portion of the foregoing chapter on Nautiloidea was revised for the first 

 edition of this work by tlie late Professor Alpheus Hyatt. In the earlier edition some nine- 

 teen new genera of fossil Nautiloids were proposed by Professor Hyatt, as well as many new 

 genera of Ammonoids. The type species of these new genera were designated, but tlie author's 

 intention to publish suitable generic diagnoses has remained for the most part unfulfilled. 

 —Editor.] 



[Tablk 



