RANGE AND DISTRIBUTION OF TIIK XAUTIUUDKA :.:i:i 



a more or le>s inflated >heath tliat do.-ed tin- -iphunclr, ;ilnl connected it with the 



distal opening of the next succeeding M-ptum, thus completely re\er.-ing the relative 

 po-itions of funnels and sheaths in other forms. The appearance- &fl de-crjbed by 

 Barrande are not deemed Hillicieiit to prove tin- truth of tlii.- .-tatement, and it iv 

 unsafe to accept it absolutely until the development has Keen >iudied. The cavity i- 

 divided by radiating lamellae running longitudinally a.- in the 



Range and Distribution of the Nautiloidea. 



Fossil Nautiloidea have been recorded by Millings as occuri -ing in Canada earlier 

 tlian the Quebec Group, but his statement lacks confirmation An abundant GVphalo- 



podan fauna makes its appearance in the earliest Quebec or ( 'alciferou.-, and i- quite 

 distinct from other later a. -semi >1 ages. Diphragmoceras and other orthocera<-one- and 

 cyrtoceracones with very peculiar si pliancies occur here, but gyroceracones and nautili- 

 cones are absent. However, the information we have at piv-ent of tlii- fauna i.- 

 limited, and but few positive conclusions can be drawn. 



All the sub-orders of Nautiloidea are initiated in the Ordovician, and one of them, 

 Schistochoanites, is confined to this period. Holochoanites and Mixochoanite* become 

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

 01 ders that disappear at this early date are remarkable for their complicated siphun- 

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

 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 al-o 

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

 estimating the relative proportions of straight and coiled conchs. Other sonr>.~ o| 

 error are presented by sporadic uncoiled or gerontic forms which occur in I'amilie- 

 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 um oiled 

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

 straight or approximately straight >hells, as against seven families of gyroceracones and 

 nautilicones. Thereafter until toward t he close of the Palaeozoic, the proportions of 

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

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

 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 curvet] 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 probab],- 

 ancestral close-coiled type. Other families nave 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, but none have 



