68 GUIDE TO THE FOSSIL REPTILES, AMPHIBIANS, FISHES. 
Table-cases 
4 8. 
Table-ease 
6 . 
Wall-cases 
2, 3. 
Table-cases 
4, 5. 
From the Lias onwards it is easy to distinguish the sharks 
and skates. The former, of the Sub-order Asterospondyli 
(“ star-vertebrse ”), always exhibit an anal fin, and when 
the vertebrae are strengthened, radiating plates predomi¬ 
nate over concentric plates in their structure. The skates 
and their allies, of the Sub-order Teetospondyli (“ covered 
vertebrae ”), are destitute of an anal fin, and their vertebrae, 
when fully developed, are strengthened by hard concentric 
layers. 
Sub-order 1.— Asterospondyli. 
The Notidanidae, which are perhaps the most primitive sur¬ 
viving family of sharks, are represented by numerous typical 
teeth of Notidanus from Upper Jurassic, Cretaceous, and 
Tertiary formations (Table-case 6). It is noteworthy that the 
largest and most complex teeth (Fig. 65) are those from the 
Fig. 65.—Tooth of Notidanus gigas, from the Red Crag of Suffolk; nat. 
size. (Table-case 6.) 
latest deposits. The Cestraciontidae are also primitive, and re¬ 
presented only at the present day by the Port Jackson shark, 
Cestracion (Figs. 66, 67), which lives on shell-fish, and has 
crushing teeth on the sides of the jaw with prehensile teeth 
in front. To this family may probably be referred the 
Carboniferous sharks, Sphenacanthus and Tristychius , which 
have cuspidate teeth and ribbed dorsal fin-spines (Wall-case 
2, Table-case 4). The fine teeth of Orodus from the Carboni¬ 
ferous Limestone are also probably Cestraciont (Table-case 
4). Hybodus, ranging from the Muschelkalk to the Wealden, 
exhibits a persistent notochord, cuspidate teeth, and ribbed 
dorsal fin-spines (Fig. 68); many specimens, presumably 
males, are further provided on each side of the head with 
two large barbed hooklets on a broad base (originally named 
Sphenonchus). The finest specimens of Hybodus, exhibited in 
Wall-case 2 and Table-case 4, were obtained from the Lower 
Lias of Lyme Regis, Dorset, and the Wealden of Pevensey 
Bay, Sussex. Acrodus , ranging from the Muschelkalk to the 
