614 MOLLUSCA phylum vi 



Range and Distribution of the Nautiloidea. 



Fossil Nautiloidea have been recorded Ly Billings as occurring in Canada earlier 

 than the Quebec Group, but his statement lacks confirmation. An abundant Cephalo- 

 podan fauna makes its appearance in the earliest Quebec or Calciferous, and is quite 

 distinct from other later assemblages. Diphragmoceras and other orthoceracones and 

 cyrtoceracones with veiy peculiar siphuncles occur here, but gyroceracones and nautili- 

 cones are absent. However, the information we have at present of this fauna is 

 limited, and but few positive conclusions can be drawn. 



All the suborders of Nautiloidea are initiated in the Ordovician, and one of them, 

 Schistochoanites, is confined to this period. Holochoanites and Mixochoanites become 

 extinct in the Silurian, and only Orthochoanites survive the Paleozoic. The sub- 

 orders that disapi^ear at this early date are remarkable for their complicated siphun- 

 cular structure, and peculiar sigmoidal septa observed in the gerontic living chambers 

 .of certain forms (Ascoceras, Gonioceras), while their prevailing habit is gyroceraconic. 

 The sigmoidal septa do not become complicated in correlation with closer coiling of the 

 shell, but occur in cyrtoceracones correlating with highly compressed cones, and in 

 orthoceracones correlating with strongly depressed cones. 



The older classifications recognised the straight orthoceracones, curved cyrtocera- 

 cones, loosely coiled gyroceracones, and more closely coiled nautilicones as distinct 

 natural divisions. Although it is possible to employ the habit of curvature in con- 

 junction with family groups as a convenient means for tracing laws of distribution and 

 the like, yet for more accurate data the genera must be considered independently. 

 For instance, some families made up largely of gyroceracones and nautilicones also 

 contain a few orthoceracones and cyrtoceracones, and these have to be neglected in 

 estimating the relative proportions of straight and coiled conchs. Other sources of 

 error are presented by sporadic uncoiled or gerontic forms which occur in families 

 having coiled shells. In a general way, however, it is possible to state the morphic 

 succession as follows : — 



Orthoceracones, together with their almost invariably associated cyrtoceracones 

 exceed gyroceracones in the Quebec in the proportion of three families to one, and this 

 horizon contains but one family of closely coiled nautilicones, and one of the uncoiled 

 or gerontic type. In the Ordovician are found no less than fourteen families having 

 straight or approximately straight shells, as against seven families of gyroceracones and 

 nautilicones. Thereafter until toward the close of the Paleozoic, the proportions of 

 straight and coiled forms remain approximately equal. The Permian has but one 

 surviving family of orthoceracones, and four of the coiled groups ; in the Trias the 

 ratio is one to six, and in the Jura coiled forms alone persist. Thus, a slowly working 

 tendency is apparent, leading to the production of more and more closely coiled cones, 

 and the elimination of straight and slightly curved forms. Gyroceracones disappear 

 with the Carboniferous, and the more discoidal nautilicones with the Trias. 



Some curious features are presented by the phylogerontic or uncoiled shells. Only 

 one family, the Silurian Lituitidae, have all the genera uncoiled save the probable 

 ancestral close-coiled type. Other families have isolated genera or species exhibiting 

 similar tendencies, and becoming partially uncoiled during their later stages, although 

 close-coiled in the young. Such forms occur throughout the Devonian, Imt none have 

 yet been found in the Carboniferous, where uncoiling of the volutions, when it 

 occurred, took place earlier than the gerontic stage. From the ]\Iesozoic 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 



