268 Address to the i-incolnshive Naturalists’ Union. 
Amongst the mollusca the Ammonites hold the first place : 
Chambered shells of great beauty, which have their counterpart 
in the Nautilus of the present day, they vary very much in shape, 
and are so distinct, that they have been used to designate zones 
of life in describing the Liassic strata, each zone having its 
distinct Ammonite as a characteristic feature; and, although 
this cannot altogether be relied on,—some Ammonites being 
found in more zones than one, and not always in the zones to 
which they give their name,—yet the fact of different species 
being found in succession one above the other as the higher beds 
appear, bears strong testimony to the vast period of time that 
must have elapsed during the formation of these strata. We 
have only to call to mind how slowly forms of molluscau life 
(and we may say the same of life generally), die out now, and are 
replaced by others, to appreciate this. 
Taking an illustration near our own time, we find that, out 
of the shells in the Norwich Crag at the top of the Pliocene 
period in the Tertiary age, 85 per cent exist at the present day ; 
and yet, between that period and our own, lies the whole of the 
Pleistocene and Glacial age, during which the Mammoth, the 
Cave Bear and the Hyena, the Woolly Rhinoceros, the great Irish 
Elk, and other animals, appeared on the scene and passed away ; 
hunted to death for the most part by man. 
It is however in the Saurians that the great interest of this 
period centres. Huge fish-like lizards from 20 to 30 feet long,— 
Icthyosaurs, with eyes 14 inches in diameter, and Plesiosaurs, 
with long swan-like necks,—infested the shallower gulfs and 
bays; some swimming out in the open water and feeding on the 
fishes and Ammonites, others hiding themselves amongst the 
tangle and in the crevices of the rocks, and darting out at their 
passing prey, : 
‘Dragons of the prime 
‘That tare each other in their slime.” 
while Pterodactyls,—large, flying, bat-like lizards, which are 
principally found in the higher Jurassic strata,—pursued their 
victims in the air, and clung to the cliffs and rocks on shore. A 
