i:^TRODrCTIOX. 
XV 
The partly coiled, partly uncoiled forms, such as Lituites^ Ophi~ 
dioceras, and Ancistrocems, in which the last whorl is produced into 
a straight line, sometimes {Lituiies and Ancistroceras) of great 
length, are generally regarded as showing a retrograde condition of 
development, with a tendency to revert to the straight, or slightly 
curved, ancestral type. Some other forms — Hercocems, Trocho- 
ceras, and Adelplioceras — present various anomalies of structure. 
In llercocems, which is a Xautilus-like shell, the aperture is very 
contracted owing to the incurvation of its margin ; in Trochoceras 
the whorls have a tendency to be helicoidal, that is, they are not 
coiled in the same plane^ ; and in the allied genus Addphoceras the 
shell is also asymmetrical, with a very marked contraction in the 
aperture. Further consideration of these and some other forms 
(^Gyroceras, Trocliolites^ (kc.) must be, for the present, postponed, os 
the exigencies of space preclude the completion of the Xautiloidea 
in the present volume. 
Attention may now be directed to the distribution of the 
Xautiloidea in space and time. 
Barrande has laid much stress upon the fact of the very sudden 
appearance of the Cephalopods in the Palceozoic rocks, under various 
generic and specific types. Thus we have in Britain an Orthoceras 
and a Cyrtoceras appearing in the Tremadoc of Wales : in Canada 
the first occurrence of Cephalopods is marked by two fragments of 
Orthoceras (?) from the upper part of the Potsdam Sandstone ^ 
(Lower Lingula Flags) : in Bussia several types of Cephalopods 
make their appearance in tho Oy’^/ioc^ms-Limestone, whilst in 
Bohemia the earliest types are met with in bande D 1 (Lower 
Ordovician). Although so rare in the Cambrian, Cephalopods begin 
to appear in considerable numbers in the Ordovician, Barrande 
having recorded (1870) in this System 474 species for all countries. 
Of these more than one half (261 species) belong to the genus 
Orthoceras. Cyrtoceras takes the second rank with 93 species, 
then follow Lituites with 45 species, and Endoceras with 41 species. 
The numbers of species of other genera — viz. : Conoceras., Filoceras, 
Gomphoceras^ Discoceras, “ Nautilus,'’ Gonioceras, Tretoceras, Tro- 
choreras, and Ascoceras — occurring in the Ordovician are very 
insignificant compared with those just named. It will be under- 
^ Had not this asymmetry been observed in a great number of species, it 
might have been set down to pathological causes. 
^ Geol. of Canada, 1863, p. 102, figs. 10, 11. 
