PLAOOGANOIDEI. 151 



would have vainly pursued in open sea, but may have been 

 euabled also to present to its enemies, when assailed in its 

 turn, only its armed portions, and to protect its .unarmed 

 parts in its burrow."* 



Family. — Stukionidjs. 



The Sturgeons are an exceptional kind of fishes at the 

 present day. They include one of the few existing genera 

 (Sturio) which have the " ganoid " scales, but these have the 

 size and shape of plates, joined by suture on the head, de- 

 tached in rows along the trunk. This placoganoid type 

 of exoskeleton is combined with as ancient a condition of 

 the vertebral column, in which the notochord is persistent 

 and the vertebral bodies consequently absent, whilst the for- 

 mation of the arches and their appendages does not pass 

 beyond the cartilaginous stage, except in parts of the haemal 

 arches of the skull. The other genera of the family shew the 

 exoskeleton either in excess, so as to encase the caudal part 

 of the trunk (Scaphyrhynehus), or almost wanting, as in the 

 paddle-fish of North America {8 r patidaria). The whole family 

 is edentulous. The skeletal basis of the head and fins is, 

 however, sufficiently hard to be preserved in the fossil state ; 

 and thus fishes allied to the last-named aberrant genus have 

 become known to us as having tenanted the liassic seas of 

 (now) British coasts (Lyme Eegis, Whitby). The name 

 Chondrosteus was given to this genus by its discoverer, 

 Agassiz ; two species have been well described and figured 

 by Egerton.f 



In the arrangement of the cranial plates, and of the 

 edentulous maxillary and mandibular arches, in the per- 

 sistent notochord, in the apparent composition of the neur- 

 apophysis of two pieces, and in the confluence of the 



* Hugh Miller, Rambles of a Geologist, p. 288. 

 f Philos. Trans. 1858, p. 871. 



