4:50 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. V. 



pieces in close juxtaposition and united by fine sutures. There are 

 fifteen fossil species of the Myliobatis (Eagle-ray), in some of which the 

 dental plates are of great size, as shown by specimens in the Table-case 

 under review. There are likewise some fine examples of the allied 

 genus, JEtobatis, from Bracklesham. 



ICHTHYODORULITES, or Fossil Dorsol-fin-spines. Table-cases, 2, 4, 6. 

 The fossils thus named are the rays or spines of the dorsal fins of cartila- 

 ginous fishes ; of these, there is an extensive series in the collection, as 

 specified in the List of Genera. I would direct attention to some beau- 

 tiful specimens in the Table-case 2, and especially to the large spine of 

 Ptychodus spectabilis in Chalk, formerly in my collection, and one of 

 the most interesting fossils of this kind hitherto discovered. 



GEOLOGICAL DISTRIBUTION OF FOSSIL FISHES. From the incidental 

 notices of the geological habitats of the Ichthyolites described in the 

 preceding pages, the intelligent reader cannot fail to perceive that 

 the most recent deposits abound in forms allied to the existing genera 

 and species, while the most ancient teem with unknown families, which 

 are either utterly extinct, or of excessive rarity at the present time. 

 M. Agassiz affirms that in a general sense we may conclude that the 

 Ichthyolites of the Tertiary deposits approach in their character to the 

 living genera, but all the species are extinct. That the newer Tertiary, 

 as the Crag, contain genera common to tropical seas, as the large sharks 

 (Carcharias), and eagle-rays (Myliobates), &c. In the Eocene, as the 

 London and Paris basins, Monte Bolca, &c. owe-tliird of the Ichthyolites 

 belong to extinct genera. Of the Chalk fishes, ^wo-thirds are of extinct 

 genera, but related to those of the Tertiary formations. From the 

 Oolite to the Lias, including the Wealden, the fishes constitute a natural 

 group, but few species of which occur in the Chalk, and all the ganoid 

 fishes are homocercal; below the Lias, a prodigious number of unknown 

 genera and species appear, and these are almost all heterocercal. 



Thus of the eight thousand living fishes known to naturalists, three- 

 fourths belong to the Cycloid and Ctenoid orders, and of these no species 

 occur below the Chalk ; the other fourth is referable to the Placoids and 

 Ganoids, of which there are comparatively but few existing species. 

 Yet fishes of these two orders were almost the sole representatives of the 

 Class Pisces, during the ancient secondary formations ; for below the 

 Lias, the predominant recent orders are altogether absent. Beneath the 

 Coal true carnivorous fishe?, with trenchant teeth, are almost unknown ; 

 omnivorous species, with either brush or obtusely conical teeth, and 

 great sauroid fishes, are the prevailing representatives of the class. In 

 fine, the Ichthyolites of the different formations constitute two grand 

 groups, which have their boundary line at the base of the Cretaceous 

 deposits. The first and most ancient comprises the Ganoids and Pla- 

 coids ; the second is more intimately related to existing types, and com- 

 prehends forms more diversified ; these are principally Ctenoids and 

 Cycloids, with a small number of the two preceding orders, which insen- 

 sibly disappear, and their few living analogues are very distinct irona the 

 ancient species. 



Now, although deductions of this nature may require to be modified 

 with the progress of knowledge, yet the generalizations thus obtained 

 are founded on so great an accumulation of facts and observations, as to 



