350 THE ORIGIN OF VERTEBRATES 
and it is difficult to believe that an olfactory opening would not be 
seen if any such had existed, as it does in Thyestes. 
The difficulty of interpreting these types is the difficulty of under- 
standing their method of locomotion ; that is largely the reason why 
the spine has been placed as if projecting from the back, and a fish- 
like body with a heterocercal tail-fin added. If, on the contrary, the 
2) 
Fig. 142, Restoration oF Preraspis. (After Smita Woopwarp.) 
spine is a terminal tail-spine, then, as far as the fossilized remains 
indicate, the animal consisted of a dorsal shield, a ventral shield, and 
a tail-spine, to which must be added two apparently lateral pieces 
and a few scales. If the animal did not possess a flexible body with 
a tail-fin, but terminated in a rigid spike after the fashion of a 
Limulus-like animal, then it must have moved by means of 
