XIV 
INTRODTJCTION. 
Orthoceras heading the list with a stratigraphical duration extending 
from the Cambrian to the Trias. Cyrtoceras which, so far as is 
known, appeared simultaneously with Orthoceras, or perhaps a little 
before it, existed for a slightly shorter period, viz. from the Cam- 
brian to the Permian. These two genera attained their highest 
development in point of numbers and diversity of form in the 
Silurian. The limitations put upon the duration in time of the 
types here mentioned must, however, be received with great caution, 
because those probable “ breaks in the geological record ” which an 
unknown amount of denudation of rock surfaces has given rise to, 
must always restrain us in any attempt to draw hard-and-fast lines 
in assigning limits to the duration of fossil forms. 
It was Barrande’s opinion that Orthoceras and Cyrtoceras are 
connected with each other through the “brevicone” forms of the 
first-named genus, and Hyatt considers that the Gomphoceratidcs 
were derived from Cyrtoceras through the “ lower forms ” of Gom- 
'phoceras (with wide apertures), which are abundant in the Niagara 
group of North America b Poterioceras, which has a simple aper- 
ture, might perhaps with even more probability be assumed to be 
the connecting form between Cyrtoceras and Gomphoceras. Next 
in order comes a very remarkable type, Mesoceras^, which Barrande® 
considered of great importance in connection with the question of 
the gradual development of the parts of the shell of the Nauti- 
loids and the mutual relations of the genera. The form of the 
aperture in Mesoceras is not simple, because it does not conform 
to the plane of the transverse section, neither is it complex as in 
Gomphoeeras, because it has only a single orifice ; most palaeon- 
tologists therefore would regard its aperture as something transi- 
tional between a simple and a complex one. This is the opinion 
held by Barrande, who, however, draws attention to the fact that 
Mesoceras does not make its appearance until an epoch of the 
Bohemian Silurian is reached in which 60 species of Gomphoeeras, 
with characteristically contracted and complex apertures, flourished, 
associated with a host of species of Orthoceras, with equally cha- 
racteristic simple apertures. It would be rash to attempt to draw 
conclusions as to the derivation of this exceedingly aberrant and 
rare form upon the single specimen known, which consists only of 
a cast of the body- chamber. 
^ See Tetrameroceras Conradi, ^at p. 241 of this volume. 
* See infra, p. 244. 
* Syst. Sil. de la Boh5me> vol. ii. Texte t. 1877, p. 1422. 
