2 
"Boos V.] TYPE FOSSILS. _ 615 
the fossils found in a formation must be older than those in the 
formation above, and younger than those in that below. This order, 
however, must be first accurately determined ; for so far as regards 
organic structure or affinities, there may be no discoverable 
reason why a particular species should precede or follow another. 
Unless, for example, we knew from observation or testimony that 
Rhynchonella plewrodon is a shell of the Carboniferous Limestone, and 
Rhynchonella tetrahedra is a shell of the Lias, we could not, from 
mere inspection of the fossils themselves, pronounce as to their real 
geological position. It is quite true that by practice a paleontologist 
has his eye so trained that he can make shrewd approximations to 
the actual horizon of fossils which he may never have seen before 
(and this is more especially true in regard to the mammalia, as will 
be immediately adverted to), but he can only do this by availing 
himself of a wide experience based upon the ascertained order of 
appearance of fossils as determined by the law of superposition. 
For geological purposes, therefore, and indeed for all purposes of 
comparison between the faunas and floras of different periods, it is 
absolutely essential first of all to have the order of superposition of 
strata rigorously determined. Unless this is done the most fatal 
mistakes may be made in paleontological chronology. But 
when it has once been done in one typical district, the order thus 
established may be held as proved for a wide region where, from 
paucity of sections, or from geological disturbance, the true succession 
of formations cannot be satisfactorily determined. 
The order of superposition having been determined in a great 
series of stratified formations, it is found that the fossils at the bottom 
are not quite the same as those at. the top of the series. As we trace 
the beds upward we discover that species after species of the lowest 
platforms disappears, until perhaps not one of them is found. With 
_ the cessation of these older species others make their entrance. 
These in turn are found to die out and be replaced by newer forms. 
After patient examination of the rocks, it is ascertained that every 
well-marked formation is characterized by its own species or genera 
(type-fossils, Leitfossilien) or by a general assemblage or facies of 
organic forms. This can only, of course, be determined by actual 
practical experience over an area of some size. The characteristic 
fossils are not always the most numerous; they are those which occur 
most constantly and have not been observed to extend their range above 
or below a definite geological horizon or platform. As illustrations of 
the type-fossils characteristic of some of the larger subdivisions of the 
Geological Record, the following may be given. Lepidodendra 
and Sigillaria are typical of Old Red Sandstone and Carboniferous 
formations; Graptolites of the Silurian system; Trilobites of Paleozoic 
rocks from Cambrian to Carboniferous, but most especially of 
Silurian formations ; Cystideans of the older Paleozoic formations ; 
Orthoceratites of Paleozoic and Ammonites of Mesozoic formations; 
Ichthyosaurs and Plesiosaurs of Mesozoic formations; Nummulites, 
_ 
