viir SKULL 2 1 I 



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 : — (i.) Dermal or mem- 

 hrane hones. 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 tones, 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, (ii.) Cartilage hones. 



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 (Sahno 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. 



