LORQUINIA 
35 
The skull of a mammal may be said to include the cranium or brain 
box and with it the bones of the face ; the lower jaw or mandible, the 
hyoid bone, styloid process and the bones of the middle ear. 
The shark's skull is a loose, cartilaginous affair and is very valuable 
to us in affording a clue to the working out of the vertebrate idea. The 
shark possesses seven gill-arches and the first and second of these play 
an important part in the formation of the skull. 
The first gill-arch divides in the middle and form what are known 
as the palato-quadrate and Meckel's cartilages. The former is homo- 
logous to the tooth-bearing bones of the upper jaw of mammals and 
the latter corresponds to the mandible or lower jaw of the higher verte- 
brates, with this difference that, in the lower forms, as sharks, Meckel's 
cartilage is the lower jaw and in the higher vertebrates it is the core or 
rod about which the bony material is deposited. 
In these higher forms Meckel's cartilage disappears at an early 
time and is therefore not found in the adults. It is to be seen in such 
reptiles as the alligator. 
A small part of Meckel's cartilage is constricted off at an early 
period in the embryo of mammals and becomes the anvil of the middle 
ear. The other two bones of the middle ear, the incus and the stapes, 
are derived from the second gill-arch which also forms the hyoid bone. 
Upon or in the two jaws, the upper and lower, the teeth are formed 
and in both cases they are derived from the epidermis. 
The teeth of al vertebrates are divided into two groups — homodont 
in which all are alike and cone-shap and heterodont — in which there 
are four kinds, incisors, canines, premolars and molars. 
Farther than this we will not discuss teeth more than to say that 
they are very important as factors in classification. 
As early as ihe Devonian age, there appeared peculiar, generalized 
fishes known as the Dipnoi, in which the swim-bladder began to be 
modified as an organ of respiration. Along with this comes a modifica- 
tion of the limb structure suggesting the legs of the amphibians. The 
heart also is modified in keeping wth the new method of respiration. 
Probably in the Devonian, and certainly in the Carboniferous, there 
appeared great, armor-headed amphibians called stegocephals. These 
are the earliest amphibians of which we have any record. 
In this animal we find the first of the middle-ear bones, the stapes 
(derived from the second gill-arch) and two occipital condyles, which 
is exceptional below mammals, which regularly possess two occipital 
condyles. 
As the name would indicate the skull of these early amphibians was 
entirely covered by bony plates — that is, it was not open at the top as we 
see in so many other of the simple vertebrates. Cavities for the eyes 
and nostrils were present and also one for the pineal eye. 
The quadrate bone which, in birds and reptiles is placed between 
