Chap. IX. 
GEOLOGICAL KECOKD. 
289 
by the percolation of rain-water. I suspect that but few 
of the very many animals which live on the beach 
between high and low watermark are preserved. For 
instance, the several species of the Chthamalin® (a sub¬ 
family of sessile cirripedes) coat the rocks all over the 
world in infinite numbers: they are all strictly littoral, 
with the exception of a single Mediterranean species, 
which inhabits deep water and has been found fossil in 
Sicily, whereas not one other species has hitherto been 
found in any tertiary formation: yet it is now known 
that the genus Chthamalus existed during the chalk 
period. The molluscan genus Chiton offers a partially 
analogous case. 
With respect to the terrestrial productions which 
lived during the Secondary and Pateozoie periods, it is 
superfluous to state that our evidence from fossil 
remains is fragmentary in an extreme degree. For 
instance, not a land shell is known belonging to 
either of these vast periods, with the exception of one 
species discovered by Sir C. Lyell and Dr. Dawson in 
the carboniferous strata of North America, of which 
shell several specimens have now been collected. In 
regard to mammiferous remains, a single glance at the 
historical table published in the Supplement to LyelFs 
Manual, will bring home the truth, how accidental and 
rare is their preservation, far better than pages of detail. 
Nor is their rarity surprising, when we remember how 
large a proportion of the bones of tertiary mammals 
have been discovered either in caves or in lacustrine 
deposits; and that not a cave or true lacustrine bed is 
known belonging to the age of our secondary or palaeo¬ 
zoic formations. 
But the imperfection in the geological record mainly 
results from another and more important cause than any 
of the foregoing; namely, from the several formations 
o 
