288 Animal Life 
feature of these teeth is their mode of succession, the new ones beimg 
developed on the under-surface of the bone, and being gradually brought 
into use by its rotation as those on the opposite edge are worn away 
and shed (Fig. 3). It results from this peculiar mode of succession, 
which is very similar to that of some of the sharks, as described in 
the preceding article of this series, that the germs are situated imme- 
diately below the teeth in actual use, but, unlike the successional teeth 
wee eon or the Of crocodiles, with their crowns facing in the opposite direction. These 
teeth in Lepidotus, teeth must have belonged to huge fishes of six or seven feet in length, 
Atthe top is a tooth and, indeed, entire specimens of these approximate dimensions haye been 
in use, while below 3 p 4 : 
on the right is its discovered in the corresponding formations of the Continent. 
Here oan oe ae Whether these fishes were carnivorous or herbivorous I have no 
means of determining. The Wealden species, which was the last of its 
kind, was a fresh-water fish. On the other hand, judging from the associated fossils, 
the giant lLepidotus was marine. Apparently this indicates that a decadent type, 
originally marine, obtaimed a temporary refuge in fresh-waters (where it became dwarfed 
in size) before its final extinction. 
From those of Lepidotus there is an easy transition to the teeth of the Oolitic 
and Liassic group known as Pycnodonts, or thick-toothed fishes. Like Lepidotus, these 
enamel-scaled pycnodonts were deep-bodied fishes, somewhat recalling a John Dory in 
shape. The teeth on the palate, and more especially on the bones known as the vomers, 
as well as those on the lower Jaw, are arranged in a number of longitudinal rows, 
more numerous in the lower than in the upper jaw. From this regularity in their 
mode of arrangement, coupled with the elegant sculpture on the crowns of some, the 
teeth of the pycnodonts are some of the most beautiful of all fossils. Im the typical 
Pycnodus (Fig. 4) the teeth haye smooth and often transversely elongated crowns, those 
on the vomer forming a central row of broad ones flanked by two rows of narrower 
ones, while in the lower jaw there is a gradual diminution in size from the two middle 
rows to the outer ones. On the other hand, in the genus Gyrodus (Fig. 5), as well 
as im several allied types, the crowns of the teeth are beautifully fluted, sometimes 
concentrically and deeply, and sometimes more irregularly. These pavements of thick, 
round, convex or flattened teeth characteristic of the pyenodonts, wrote Sir Richard 
(then Professor) Owen many years ago, are adapted, like the corresponding teeth of 
certain existing fishes, to crush small molluscs and crustaceans. Whether this is anything 
more than an inference I am unaware; but evidence is, I believe, afforded that these 
fishes were carnivorous from the occurrence within their fossilised remains of scales of 
other members of their tribe. All of them were apparently marine. 
Much more might be written with regard to the dentition of these and other 
types of extinct enamel-scaled fishes, but were I to devote more space to this group 
I should have none remaining for the bony fishes of the present day and their extinct 
relatives, some of which possess most remarkable dental adaptations of their cwn. 
As one of the most aberrant and remarkable, 1 may 
commence with the curious Globe-Fishes, or Porcupine Fishes, 
whose small bodies are sparsely covered with spines, and 
when inflated (as they can be at the will of- their owners) 
look lke spiny footballs. In these tropical fishes the whole 
front of the jaws is formed by a great cutting-tooth, which 
in one genus, hence called Tetrodon, is divided in the middle 
line, both above and below, into two halves (Fig. 6), while in 
the other (Diodon) it is continuous in each jaw. In the 
Tig. 4. Palatal Dentition of 2 S : 
TDCI. former there is a rudimentary dental plate on the palate and 
