RANGE AND DISTRIBUTION OF THE NAUTILOIDEA 533 
a more or less inflated sheath that closed the siphuncle, and connected it with the 
distal opening of the next succeeding septum, thus completely reversing the relative 
positions of funnels and sheaths in other forms. The appearances as described by 
Barrande are not deemed sufficient to prove the truth of this statement, and it is 
unsafe to accept it absolutely until the development has been studied. The cavity is 
divided by radiating lamellae running longitudinally as in the Actinosiphonata. 
Range and Distribution of the Nautiloidea. 
Fossil Nautiloidea have been recorded by 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 
eyrtoceracones with very 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 sub-orders 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 Palaeozoic. The sub- 
orders that disappear 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 oceur 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 Palaeozoic, 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 phylogerontie or uncoiled shells. Only 
one family, the Silurian Litwitidae, 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, but none have 
