ORDER ELASMOBRANCHEI. 925 



plied to apertures connected with respiration found on the upper 

 aspect of the head ; while the intercalary cartilages are ovoid or 

 diamond-shaped structures occurring between the neural arches of 

 the vertebrae. 



Mr Smith- Woodward observes that a gradual advance in the de- 

 gree of calcification of the axial skeleton may be observed as we trace 

 the members of the order from the Palaeozoic upwards, the oldest 

 forms not having the notochord differentiated into distinct vertebral 

 centra, of which we find the first complete types in the Lower Liassic 

 Palceospinax. Here, however, the centra merely assume the form 

 of double cones, only very slight traces of the peripheral calcifica- 

 tions necessary to form the biconcave centra of the later types being 

 visible. In the Lower Kimeridgian lithographic stone of the Con- 

 tinent, centra of the complete asterospondylic type are first met with. 



The same authority mentions that the Palseozic types are, as a 

 rule, characterised by the great development of the exoskeleton, 

 although a few forms like Pleuracanthus appear to have had naked 

 bodies. These early dermal structures are also noticeable for their 

 elaborate sculpture, this being equally developed on the dermal 

 granules (shagreen), and on the fin and cephalic spines. Smooth 

 fin-spines appear to be very rare in the Palaeozoic, and it is not till 

 we reach the Upper Trias and Lias that spines completely covered 

 with smooth ganoine are met with. 



In regard to the teeth, Mr Woodward observes that " pointed 

 teeth and obtuse teeth occur among the earliest Elasmobranchs ; 

 but the former as well as the latter are firmly articulated together, 

 and must always have formed part of a dentition in which several 

 series were functional. Though the teeth of Cladodus and Diplo- 

 dus [Pleuracanthus\axe as sharply pointed as those of most recent 

 Sharks, the piercing crown is placed upon a broad horizontally- 

 expanded base, permitting of a considerable amount of interlock- 

 ing between one tooth and another — an arrangement most nearly 

 paralleled in the surviving Chlamydoselache. It is evident, indeed, 

 that all the modern types of dentition, in which not more than one 

 or two series of teeth are simultaneously functional, are highly speci- 

 alised modifications of this primitive arrangement ; and the change 

 results from the deepening and lateral compression of the root of 

 each tooth, rendering its base of support less fixed, and often not 

 permitting its coming into use until after attaining the summit or 

 passing the outer side of the jaw-cartilage. 



" With regard to the disposition of the teeth in the mouth as a 

 whole, the modern Rays — most Scylliidce and Chlamydoselache — may 

 be looked upon as retaining the most primitive arrangement. In 

 the predaceous Sharks there has been a tendency towards the rela- 

 tive enlargement of the prehensile teeth upon the symphysis ; while 



