XVlll 
INTRODUCTION. 
by its ventral sutures with the Ammonoidea through Mimoceras, 
Hyatt. Again, in the Clymenidse and Goniatitidae of the Devonian 
and the Carboniferous and in the Ceratitidae 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 Goniatitidae, 
which are in many respects Nautiloid in their characters \ made 
their first appearance in the Devonian, and therefore after the 
Hautiloidea had attained their maximum of development, which 
took place in the Silurian epochs 
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 results of recent discoveries relating to the structure of the 
shells of Endoceras^ Piloceras, Actinoceras^ and Ascocems 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. 334), at the end of the volume. 
^ Hyatt, Proc. Boston Soc. Nat. Hist. vol. xxii. 1883, p. 307. Meek 
(United States Geol. Surv. Terr. vol. ix., 1876, footnote, p. 491) makes the 
following interesting remarks bearing upon the same subject under the de- 
scription of his subgenus Pseudonaictilus, in reference to which he says ; — 
“ This type presents very curious and interesting intermediate characters 
between the Nautihcs and Goniatites groups ; that is, it has not only the lateral 
lobes of the septa of the latter, as we see in some other Nautiloid groups, but 
also the peripheral, or so-caUed dorsal lobe of Goniatites, while its siphon is 
removed from the peripheral margin, as in the Nautiloid forms. Still it is so 
near the outer margin that it would only have to be placed two or three times 
its own breadth farther out to bring it against the peripherj", and thus make 
the shell a Goniatite, as that genus is usually understood. Such examples 
show how gradually the distinctions between genera and larger groups are 
found to fade away as our knowledge of the intermediate types extends.” 
^ See Heilprin, ‘ Geographical'and Geological Distribution of Animals,’ 1877, 
p. 265 et seq. 
® Geol. Mag. new series, Dec. ii. vol. iii. 1876, p. 249. 
