vill SKULL AAT 
the embryonic cartilaginous cranium becomes complicated by the 
addition of an extensive series of investing membrane bones, formed 
by the ossification of the connective tissue external to the cartilage, 
so that a secondary bony cranium is formed external to the primary 
cranium much in the same way that a secondary pectoral girdle 
is formed in connexion with the primary girdle. Such bones 
probably owe their primary origin to the fusion and insinking of 
exoskeletal structures (scales or dermal spines). To these invest- 
ing bones there may also be added a series of bones formed by 
the actual conversion of the cranial cartilage into osseous tissue 
(cartilage bones), which to a greater or less extent in different 
Fishes replaces the original cartilage. The bones of the skull 
may conveniently be classified as follows :—(.) Dermal or mem- 
brane bones. Under this head are included—(a) the ordinary 
investing bones of the skull. (b) Tooth-bones, that is, bones 
formed by the fusion of the bases of teeth and developed in 
‘relation with the walls of the oral cavity. Probably all tooth- 
bearing bones are of this nature. (c¢) Sensory canal bones, that 
is, tubular bones developed round the sensory canals of the head. 
Certain of these bones may secondarily acquire the shape and 
character of investing bones while still retaining protective 
relations to their sensory canals. (i1.) Cartilage bones. 
As an easily obtainable example of a skull which has acquired 
a fairly complete series of both cartilage- and membrane-bones, 
while retaining a well-developed primary cranium, the skull of 
the Salmon (Salmo salar) may be described.’ At an early stage 
of development, even so late as the second week of hatching, the 
primary cranium is still entirely cartilaginous, and in this con- 
dition the Salmon’s skull is comparable with that of an adult 
Dog-Fish. As development proceeds the primary cranium 
becomes supplemented by the addition of numerous investing 
‘dermal bones which form the secondary cranium, and _ later 
cartilage bones appear and, to a considerable extent, replace the 
original cartilage. The Salmon’s skull is interesting in this 
respect, that the primary cranium grows with the growth of the 
Fish, so that in the adult the nasal, ethmoidal, and prenasal 
regions are entirely cartilaginous, and in the hinder part of the 
cranium cartilage is largely persistent between the cartilage bones. 
Dealing first with the cartilage bones of the primary cranium, 
1 W. K. Parker, Phil. Trans. 163, 1873, p. 95. 
