416 ZOOLOGY. 
column or series of biconcave vertebra, with the cartilage in 
part replaced by bone, forming radiating leaves or plates ; 
while in the rays or skates the anterior part of the column 
is bony. 
The ribs are small, sometimes rudimentary. The skull is 
rudimentary, without membrane-bones, embryonic in char- 
acter, forming a simple cartilaginous brain-box, without pre- 
maxillary. or maxillary bones, the constitution of the jaws be- 
ing quite unlike that of the bony fishes, the jaws being formed 
‘of elements, 7. ¢., ‘‘ cartilaginous representatives of the pri- 
mary palatoquadrate arch and of Meckel’s cartilage.” (Hux 
ley.) 
There are no opercular bones such as cover the gill-open- 
ings in bony fishes, their place being taken by cartilaginous 
filaments. 
The mouth is armed in most sharks with numerous sharp, 
flattened, conical teeth, arranged in transverse rows and 
pointing backwards; they are never fixed in sockets, but 
imbedded in the mucous membrane of the upper and under 
jaws. In the Heterodontide, represented by Cestracion or 
Port Jackson shark, the teeth are much blunter than in 
other living sharks, the middle and hinder teeth having 
broad, flattened crowns, forming a pavement of rounded 
teeth. The Devonian sharks were in most cases like the 
Cestracion in this respect. In the Carboniferous age, sharks 
with teeth more like those of modern forms came into ex- 
istence ; and they must have been of a more active nature, 
the sharp teeth directed backward indicating the rapacity of 
these monsters, which darting after and seizing their prey 
were enabled to retain it by the backward-pointed teeth ; 
while the more sluggish Devonian Cestracions kept near the 
bottom and devoured the shelled mollusks, etc., possibly Or- 
thoceratites, Nautili, and Trilobites, which became nearly 
extinct about the time the type of pavement-toothed sharks 
culminated. 
The teeth of the skates or rays have obtuse points. In 
Myliobatis the teeth are flattened and united to form a solid 
pavement, so that the mouths of these large rays are fur- 
