892 



SUB-KINGDOM VERTEBRATA. 



structed of solid bony scutes, which may imbricate or overlap one 

 another, like the tiles on a roof, or may be firmly united at their 

 edges by sutural union. Such an armour may also develop enor- 

 mous bony spines, often attached to the skin by an expanded base 

 which represents the scutes. In the latter class, again, the bony 

 shells of the Testudinate Chelonia are formed partly of dermal 

 elements, blended with others belonging to the endoskeleton to 

 form a continuous whole ; while in the Athecate division of the 

 same order the whole of the protective armour is of dermal origin. 



The so-called membrane bones of the skull, and the clavicular 

 portions of the pectoral girdle, are likewise of dermal origin, and 

 therefore properly belong to the exoskeleton ; but their intimate 

 connection with cartilage bones renders it more convenient to con- 

 sider them with the endoskeleton. 



Teeth, as belonging to the list of dermal structures, may be con- 

 veniently noticed here ; and the importance of these organs to the 

 palaeontologist can scarcely be overrated, since from their extreme 

 density they are more often preserved in a perfect condition than 

 most other parts of the skeleton, and thus frequently form the only 

 safe guide to the affinities of an extinct type. Teeth are composed 

 of two or more earthy constituents, varying in their structure, and 

 the amount of animal matter contained in them. The 

 most important element is that known as dentine (fig. 

 813, d), which forms the greater portion of the body 

 of most teeth, and consists structurally of extremely 

 minute tubes, cells, and earthy particles. Some den- 

 tine is devoid of blood-vessels ; but in other cases it 

 is permeated by the latter, when it is known as vascu- 

 lar, or vaso-dentine ; and both these structures may exist 

 in a single tooth. In young teeth (fig. 813) the centre 

 of the dentine mass is occupied by the vascular pulp- 

 cavity (pc), which is open at the base ; but in the adult 

 this cavity is often totally obliterated, by the formation 

 of what is known as osteo-dentine, which is a kind of 

 vascular dentine passing imperceptibly into the structure 

 of true bone. The second primary constituent is the 

 enamel (c), which, when present, immediately overlies 

 the dentine of the crown, or exposed portion of the 

 tooth, and is the hardest known animal substance ; 

 it is composed of extremely minute prismatic fibres, 

 generally running at right angles to the outer surface 

 of the tooth. This substance is the least constant 

 element in teeth, although it is very generally present 

 in those of Mammals ; among the Reptiles it is entirely wanting in 

 the Ophidian Squamata, but is present in the Crocodilia. The 



Fig. 8i 3 .-Dia- 

 grammatic verti- 

 cal section of a 

 tooth, e, Cement ; 

 d. Dentine ; c, 

 Enamel ; pc, Pulp- 

 cavity. 



