THE VERTEBRATA 223 



this skeleton is composed of cartilage, and is there- 

 fore rarely preserved fossil. These _are sharks and rays, 

 known principally to fossil-collectors by their teeth 

 (Fig. 63), and the bony " ichthyodorulites " which 

 support the front margin of the fins in some cases. 

 Ichthyodorulites (Onchus) accompany skin-teeth in the 



FIG. 62. LANARKIA SPINOSA, TRAQUAIR, DOWNTONIAN, 



LESMAHAGOW. ( x |.) 



(After Traquair.) 



This differs from its contemporary, Thelodus, in having spiny skin-teeth. 

 The head and trunk have been flattened down from above, but the tail 

 is turned over on its side. 



Ludlow bone-bed, Shark's teeth are common in some 

 beds in most geological systems. They are usually 

 sharp lacerating teeth, pointing inwards on the jaws, 

 so as to prevent prey from slipping out ; they are 

 arranged in rows, new rows developing on the inside 

 of the jaw throughout life, and moving forwards as the 

 older teeth fall out. Thus loose teeth are commoner than 



