FOSSILS 97 
tarely been preserved. As these tracks and castings occur chiefly in 
marine sedimentary rocks, it is very doubtful if any of them indicate 
earthworms. The “tubes” formed by many marine annelids are often 
met with as fossils. 
Molluscotdea.—These are among the commonest and most abundant 
fossils. One great division (Polyzoa) comprises the lace-corals and sea- 
mats, which are chiefly marine, and, as fossils, often occur associated 
with other marine organisms. The other great division (Brachiopoda) 
is exclusively marine, and includes the lamp-shells, etc.—one of the most 
important types of life with which the student of historical geology has to 
deal. 
Mollusca.—The same holds true with the marine mollusca, which are 
more or less abundantly represented in every great system of strata. 
Not only are they of prime importance by reason of their abundance as 
regards genera, species, and individuals, but their shells, like those of the 
brachiopods, appear often in a comparatively perfect state of preserva- 
tion. Freshwater shells and land-snails are of much less frequent 
occurrence as fossils. 
Arthropoda——This phylum embraces lobsters, crabs, scorpions, 
spiders, centipedes, and insects, and is of great value to the geologist 
—the crustaceans more especially, for a large’ proportion of these 
being marine, they are well represented by fossils. Some of the extinct 
types, as Trilobites, for example, are characteristic fossils of the older 
geological systems. Freshwater and terrestrial forms are not so 
commonly encountered, since they are largely confined to freshwater 
deposits and to lignite- and coal-bearing strata. 
Vertebrata.—This great phylum is most numerously represented by 
marine fishes. Marine types of reptiles and mammals also occur now 
and again, but with the exception of the fishes vertebrate remains of any 
kind are sparingly met with. Remains of birds and land-mammals are 
almost confined, as might have been expected, to freshwater and terrestrial 
accumulations. 
Importance of Fossils in Geological Investigations.—It 
need hardly be said that the study of fossils to the biologist 
is of surpassing importance. Such study, indeed, cannot be 
ignored by him if he would understand the life-history of 
existing types. But it is not with that side of paleontological 
inquiry that the practical or field-geologist is mainly con- 
cerned. He values fossils chiefly for the help they yield him 
in his endeavours to realise the conditions under which 
sedimentary rocks were formed, and to ascertain the chrono- 
logical sequence of the strata. 
Climatic Conditions deduced from Fossils.—Individual 
fossils, if of existing species, and occurring zz sztu, may give 
valuable evidence as to former climatic conditions. Two 
G 
