The Fossil Cephalopoda. 360 
third point of attachment is to the bottom of the shell by the 
posterior extremity of the mantle, which probably presents a conical 
form in the embryo Nautilus, he continues, if then, the siphuncle 
in the young stage forms the main point of attachment between the 
animal and its shell, we may reasonably argue that the siphuncle in 
the adult Nautilus is simply the evidence of an aborted embryonal 
organ whose function is now fulfilled by the shell-muscles, but 
which in the more ancient and straight-shelled representatives of 
the group (the Orthoceratites) was not merely an embryonal, but an 
important organ of attachment between the animal and its shell in 
the adult. 
It has been observed, however, that the shell-muscles in Nautilus 
are incapable of bearing even a slight strain, and that the animal 
seems readily able to detach itself from its shell when injured (see 
Introduction, p. xi). 
Full justice is done by Mr. Foord in his “ Introduction” to all 
the various writers on the Cephalopoda, and the views of M. Barrande 
on the appearance in time and the distribution, both in time and 
space, of the several families of the Nautiloidea, are fully given. 
Mr. Foord quotes the opinion of von Jhering, Prof. Hyatt, and 
S. P. Woodward, that probably Tentaculites may have been the pro- 
totype of the Cephalopoda, an opinion, however, which Prof. James 
Hall does not share (p. x). 
In the search for connecting links between the Nautiloidea and 
Ammonoidea, our attention is arrested by the peculiar form Bactrites, 
which, though probably a true Nautiloid, is connected by its ventral 
sutures with the Ammonoidea through Mimoceras, Hyatt. Again, 
in the Clymenidz and Goniatitide of the Devonian and the Carbon- 
iferous and in the Ceratitidee of the Trias a very decided approach 
is made towards the complicated sutural line of the Ammonoidea. 
It is worthy of note also that the Goniatitide, which are in many 
respects Nautiloid in their characters, made their first appearance in 
the Devonian, and therefore after the Nautiloidea had attained their 
maximum of development, which took place in the Silurian epoch. 
Concerning the first appearance and distribution of the earlier 
forms of life in the European area, it has been suggested by Dr. 
Hicks that they originated in the warmer or more equatorial 
regions, where it is probable that the principal changes in their 
development took place; and that they gradually migrated north- 
wards as the thermal conditions became more favourable, owing to 
the subsidence of the land and the consequent spread of the seas in 
Northern latitudes. Here “the groups as they successively appeared 
always contained evidence of advance in development over those 
which had previously reached the areas.” 
The result of recent discoveries relating to the structure of the 
shells of Hndoceras, Piloceras, Actinoceras, and Ascoceras will be 
found under the descriptions of those genera, with the exception of 
Ascoceras, some new facts having come to light regarding that 
genus since the description of it was printed: these facts are 
detailed in the Supplement (p. 834), at the end of the volume, and 
