150 



COMPARATIVE ANATOMY. 



only part of the first arch, which is movable. The partly ossified 

 skull of the Dipnoi has just the same characters. 



§ 342. 



The cartilaginous cranium is most completely retained in the 

 Sturiones (Fig. 241) ; in the rest of the Gano'idei it is partly so ; in 

 the Teleostei, and especially in Salmo and Esox, it is more or less 

 so. In most forms the ethmoidal portion remains cartilaginous. 

 Henceforward, and all through the Vertebrate phylum, a cartilaginous 

 cranium can be made out in the earliest rudiments of the skull ; 

 howsoever modified, it may be derived from the primitive condition, 

 and may therefore be regarded as a remnant of that condition. 



The degenerations which affect this primordial cranium are chiefly 

 due to the ossifications which take place in it. Bony parts, which 

 perform the function of organs of support and protection, better than 

 cartilage does, take its place ; and the development of these bones, 



Fig. 241. Cephalic skeleton of Aoipenser sturioj after the removal of the cover- 



ing bones, r Rostrum, n Nasal cavity, o Optic foramen, tr Trigeminal foramen. 

 sp Spinous processes of the anterior portion of the vertebral column, which is fused 

 with the cranium. 2' Palato-quadrate. m Mandible. Hm. Hyoniandibular. s Sym- 



plectic. br Branchial arches, r Ribs. 



which enter into connection with the cartilaginous cranium, is clearly 

 the cause of the degeneration of the cartilaginous tissue. A higher 

 and more complete arrangement has ousted the lower one. 



Bony pieces become connected with the cartilaginous pieces of 

 the branchial skeleton, as well as with the cartilaginous cranium ; 

 in this way the whole cephalic skeleton gradually passes from a 

 cartilaginous into an osseous condition. The osseous elements which 

 go to make it up are almost all derived from the dermal structures, 

 which we met with as dermal denticles in the Selachii. Some of 

 these bones appear on the outer surface of the carti- 

 laginous cranium, and form the covering bones of the skull 



(cf. supra, §325). They are plates which are formed in the 

 integument, and owe their origin to the growth of placoid 



scales; in the Sturiones, already a number of them have 



taken up the same position as that which they retain in 



the higher divisions. 



