150 PICTURESQUE SKETCHES 



we can deduce the details of structure, or even 

 the form of the species, and the general dimen- 

 sions to which it attained. But the spines just 



alluded to are 

 very interesting 

 fossils, and re- 

 quire further de- 

 scription, since 

 remains of this 

 kind are exceed- 

 ingly numerous 

 and varied, and 

 ICTHYODORULITE. theiruseis known 



to us from the 



examination of a shark confined at present to the seas 

 which wash the shores of Australia. 



In this animal, which is called the " Port Jackson 

 Shark," the general form of the tribe to which it 

 belongs, the position of the mouth on the under side 

 of the body, the habit of extreme voracity, &c., are 

 all retained ; and in order, as it would seem, that 

 it may possess an unusual facility in turning itself in 

 the water to take its prey, (which can only be done 

 when the mouth is uppermost, and the body there- 

 fore inverted) there is added to the ordinary appa- 

 ratus of fins, a moveable one on the back, of con- 

 siderable dimensions, worked by a spine a kind of 

 mast which is inserted simply, but deeply, through 

 the skin into the flesh, and is elevated when neces- 

 sary by a muscular effort. When this spine is ele- 

 vated, it lifts with it the fin, and instantly begins 

 to act, assisting and rendering more rapid the turn- 

 ing of the whole fish. It is a curious fact, that the 



