152 PALAEONTOLOGY. 



scapula with the coracoid, Chondrosteus agrees with Sturio. 

 In the structure of the hyoid and opercular regions, in the 

 better ossification of the endoskeleton, and in the shape 

 and number of the " fulcral scales " of the tail, Chondrosteus 

 shews a nearer affinity to the Lepidoganoids. In the absence 

 of a spine-armed plate in front of each median fin, in the 

 more advanced position of the dorsal, in its deeper form and 

 smooth integument, it resembles Spatidaria. The fore part 

 of the head is too mutilated in the fossils to prove that it 

 might not also have resembled the Spatularia, in a paddle- 

 shaped production of that part. The liassic sturgeon seems to 

 have enjoyed a more tranquil existence than the modern ones. 

 The associated molluscous and radiate animals prove the 

 marine character of the waters it inhabited. The thinly 

 laminated beds of shale and limestone in which its remains 

 occur testify to the tranquil condition of the sea in which it 

 lived ; its smooth skin doubtless harmonizing in tint with the 

 muddy bottom, served to conceal it from the predatory 

 saurians with which it co-existed, so that Chondrosteus re- 

 quired neither defensive armour nor locomotive energy to 

 fulfil the functions assigned to it. 



Evidence of a true sturgeon (Accipenser) has not hitherto 

 been met with in formations of older date than the eocene 

 clay at Sheppey {Ace. toliaincus, Ag.) 



Sub-Order 2.— LEPIDOGANOIDEI. 

 Family I. — Dipterid^:. 



This family includes a few heterocercal fishes with two 

 dorsal fins, and a large anal, adding by their backward posi- 

 tion to the power of the main propelling organ — the tail. 

 The head is large and well defended by ganoid bones ; the 

 teeth are conical and sub-equal ; the scales perforated by 

 small foramina ; the notochord is persistent. 



