THE SKULL 57 



part of the head skeleton is a massive cartilaginous box (chondro- 

 cranium), enclosing the brain, and including, as a result of growth 

 and fusion, the nasal and auditory capsules. The eye capsules are 

 free, and are accommodated at the side of the cranium in an orbital 

 depression, to be seen on the skull of all vertebrates. This type 

 of structure is obviously the basis of the mammalian skull, ex- 

 cept that in the latter the cartilage mass is more nearly restricted 

 to its basal portion and at the same time is more specialized 

 in its replacement by definite bone centres. 



It is, however, in respect of the visceral arches 

 suspended from the chondrocranium that the 

 structure of the shark skull is most illuminating. 

 The first or mandibular arch forms the upper 

 and lower iaws. Its composition illustrates the 



• 1 • r 1 • • r 1 1 Fig. 33. Sha- 



phyloe:enetic basis oi the origm or the external green denticles of 



r ■, , . 1 • • 1 1 • the smooth dog- 



part of the auditory chain in the mammalian fish, Musteius; en- 

 embryo, the two bones which this part comprises 

 corresponding with the posterior ends of the two cartilaginous 

 jaws of the sharks. Such a relation establishes the fact that in the 

 history of mammals this arch has undergone a profound change of 

 function. The second or hyoid arch, though developed to an extent 

 out of keeping with the rudimentary state of the primitive tongue, 

 is nevertheless obviously homologous with the hyoid arch of a 

 mammal. In most sharks its upper portion has an important 

 accessory function in the support of the jaws and this part prob- 

 ably becomes the third bone (stapes) of the auditory chain in the 

 mammal. Following the hyoid arch are five ordinary or branchial 

 arches supporting the filaments of the gills and serving as pillars 

 of the gill apertures. These arches are the parts of an extensive 

 system out of which have been formed by specialization part of 

 the hyoid apparatus and most, if not all, of the laryngeal cartilages 

 of mammals. 



So far as the surface or roofing portion of the skull is concerned, 

 it is represented in a shark only by dermal teeth or shagreen 

 denticles (Fig, 33), uniformly distributed in the skin of the body, 

 and slightly modified in shape and size at the aperture of the mouth 

 to constitute definite teeth lining the jaws. In sturgeons and re- 

 lated fishes, however, these structures are already concentrated 



