58 Anatomy of the Rabbit 



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. 31), uniformly distributed in the skin of the body, 

 and concentrated at the aperture of the mouth into definite teeth 

 lining the jaws. In sturgeons and related fishes, however, these 

 structures are already concentrated into a definite 

 BONFS pattern of surface plates, having in general the 



same disposition as derm elements in the skull of 

 all higher vertebrates, and distributed in such a way that they form 

 art almost complete enclosure for those parts of the chondro- 

 cranium which they invest. The pattern of the dorsal elements is 

 best indicated in fossil amphibia and early reptilia, in which the 

 plan is almost diagrammatic. 



The elements of the head skeleton may be classified as follows : 



1. The CEREBRAL CRANIUM (cranium cerebrale or neuro- 

 cranium), including: 



(a) The primary cartilage skull (chondrocranium) , en- 

 closing the brain, and containing in its wall the olfactory 

 and auditory capsules (embryonic) ; 



(b) The secondary bone skull (osteocranium), replacing 

 (a) and comprising the occipital, sphenoid, ethmoid, 

 inferior turbinal, and periotic bones; 



(c) The associated derm elements, comprising the inter- 

 parietal, parietal, frontal, nasal, vomerine, lacri- 

 mal, tympanic,* and squamosal bones. 



2. The VISCERAL CRANIUM (cranium viscerale or splanch- 

 nocranium), including: 



(a) The primary mandibular and hyoidf visceral arches 



(embryonic) ; 



(b) The secondary elements, replacing (a) — the malleus, 

 incus, and stapes of the auditory chain; the hyoid 

 bone and its connections with the skull ; 



(c) The associated derm elements of the face and palate, 

 comprising the premaxillary, maxillary, zygomatic, 

 mandibular, palatine and vestigial pterygoid bones. 



*The identification of the tympanic as a derm element has been questioned. 



fThe thyreoid cartilage of the larynx and its connection with the hyoid 

 (greater cornu) are modified branchial arches, but the structure as a whole is not 

 included with the head skeleton. 



