The Student's Elements of Geology

the theory of progressive development, because, according to Professor Huxley, the genus Pteraspis is allied to the sturgeon, and therefore by no means of low grade in the piscine class.

It is a fact well worthy of notice that no remains of vertebrata have yet been met with in any strata older than the Lower Ludlow.

When we reflect on the hundreds of Mollusks, Echinoderms, Trilobites, Corals, and other fossils already obtained from more ancient Silurian formations, Upper, Middle, and Lower, we may well ask whether any set of fossiliferous rocks newer in the series were ever studied with equal diligence, and over so vast an area, without yielding a single ichthyolite. Yet we must hesitate before we accept, even on such evidence, so sweeping a conclusion, as that the globe, for ages after it was inhabited by all the great classes of invertebrata, remained wholly untenanted by vertebrate animals.

Dates of the Discovery of different Classes of Fossil Vertebrata; showing the gradual progress made in tracing them to rocks of higher antiquity.

Year Formations Geographical localities Mammalia 1798 Upper Eocene Paris (Gypsum of Montmartre).1 1818 Lower Oolite Stonesfield.2 1847 Upper Trias Stuttgart.3 Aves 1782 Upper Eocene Paris (Gypsum of Montmartre).4 1839 Lower Eocene Isle of Sheppey (London Clay).5 1854 Lower Eocene Woolwich Beds.6 1855 Lower Eocene Mendon (Plastic Clay).7 1858 Chloritic Series, or Upper Greensand Cambridge.8 1863 Upper Oolite Solenhofen.9 Reptilia 1710 Permian (or Zechstein) Thuringia.10 1844 Carboniferous Saarbrück, near Trèves.11 Pisces 1709 Permian (or Kupferschiefer) Thuringia.12 1793 Carboniferous (Mountain Limestone) Glasgow.13 1828 Devonian Caithness.14 1840 Upper Ludlow Ludlow.15 1859 Lower Ludlow Leintwardine.16

1. George Cuvier, Bulletin Soc. Philom. xx.
2. In 1818, Cuvier, visiting the Museum of Oxford, decided on the mammalian character of a jaw from Stonesfield. See also p. 347.
3. Prof. Plieninger. See p. 368.
4. Cuvier, Ossemens Foss. Art. “Oiseaux.”
5. Prof. Owen, Geol. Trans., 2nd series, vol. vi, p. 203, 1839.
6. Upper part of the Woolwich beds. Prestwich, Quart. Geol. Journ., vol. x, p. 157.
7. Gastornis Parisiensis. Owen, Quart. Geol. Journ., vol. xii, p. 204, 1856.
8. Coprolitic bed, in the Upper Greensand. See p. 299.
9. The Archæopteryx macrura, Owen. See p. 338.
10. The fossil monitor of Thuringia (Protosaurus Speneri, V. Meyer) was figured by Spener of Berlin in 1810. (Miscel. Berlin.)
11. See p. 406.
12. Memorabilia Saxoniæ Subterr., Leipsic, 1709.
13. History of Rutherglen by Rev. David Ure, 1793.
14. Sedgwick and Murchison, Geol. Trans., 2nd series, vol. ii, p. 141,
16. Sir R. Murchison. See p. 459.
17. See p. 461.

Obs.—The evidence derived from foot-prints, though often to be relied on, is omitted in the above table, as being less exact than that founded on bones and teeth.

In the preceding Table a few dates are set before the reader of the discovery of different classes of animals in ancient rocks, to enable him to perceive at a glance how gradual has been our progress in tracing back the signs of vertebrata to formations of high antiquity. Such facts may be useful in warning us not to assume too hastily that the point which our retrospect may have reached at the present moment can be regarded as fixing the date of the first introduction of any one class of beings upon the earth.

2. Wenlock Formation.—We next come to the Wenlock formation, which has been divided (see Table, p. 458) into Wenlock limestone, Wenlock shale, and Woolhope limestone and Denbighshire grits.

Fig. 536: Halysites catenularius.