214 
NATJTILOrDEA. 
significance. I have adopted Hyatt’s genera, because I believe that 
the characters of the aperture are of much higher importance in 
classification than the amount, or even the direction, of the curva- 
ture of the shell. 
The following observations by Barrande regarding the contraction 
of the aperture met with in Gomphoceras and other genera (such as 
Hercoceras \_Gyrocems] alatum, Barr., sp., and Hercoceras \_Troc7io- 
ceras] flexum, Barr., sp.) are so interesting and suggestive that I 
give here a translation of them : — “ The appearance,” he says, “ of 
the contraction of the aperture in Phragmoceras, Gomphoceras, and 
some other genera, makes it difiicult, at first sight, to imagine how 
the mollusk could have constructed its shell. It seems to us that 
this difficulty vanishes on studying the figures on our Plate 241, 
showing sections of different individuals of Hercoceras mirum. 
These sections show clear marks of the reconstruction of the external 
(ventral) area of the test, and of the absorption of the internal 
(dorsal) area, so that the moUusk, though apparently walled into its 
shell by a transverse diagram, could, nevertheless, gradually develop 
itself and lengthen its spire, like those Nautilids in which the aper- 
ture is quite open. 
“ By the application of the same process of reconstruction and 
absorption of the test, the shells of Phragmoceras, Gomphoceras, &c. 
&c., may have been constructed and successively augmented at the 
larger extremity, during the growth of the mollusk. It would 
therefore be needless to refer to the views of certain observers, who 
have supposed that the shells of Phragmoceras and other analogous 
types had a simple and widely expanded aperture, during the whole 
of the period of growth of each individual, and that the aperture 
became contracted only at the adult age. It should also be noted 
that in every species we find individuals of very different size, but 
all presenting an equally contracted aperture, conforming to the 
structure proper to each specific type.” 
BemarTcs. This genus is recorded as having a range extending 
from the Ordovician to the Carboniferous ; but the species occurring 
in the rocks below the Silurian, owing to their imperfect condition, 
must be looked upon with great suspicion, while those above that 
horizon belong apparently to other genera. 
Beginning with the Ordovician, we find that Dr. Ferdinand 
Eoemer ^ has described and figured a curved and rapidly expanding 
^ Die Foss. Fauna der Silur. Diluvial-Gesch. von Sadewitz, 1861, p. 61, 
Taf. vi. ff. 4, a, b. 
