PISCES. 
89 
Table-case 16, their snout is long and pointed, their tail is 
diphycereal (see p. 61), and their trunk is armoured with 
only four longitudinal rows of bony plates or scutes. The 
common Rhsetic teeth named Saurichthys seem to belong to 
fishes of this family. 
The Chondrosteidse, represented by Chondrosteus (Fig. 93) 
from the Lower Lias of Lyme Regis, perhaps also by the 
gigantic Gyrosteus from the Upper Lias of Whitby, are inter¬ 
mediate between the Pakeoniscidse and the modern sturgeons. 
The fine specimens exhibited show that the internal skeleton 
is identical with that of the sturgeons (Fig. 94), and that the 
jaws are reduced and toothless ; but the roof of the skull and 
the development of the rays below the gill-cover more closely 
resemble the corresponding parts in Palseoniscids. 
A few dermal scutes identical with those of the existing 
sturgeon, Acipenser, are shown from the English Eocene. 
There are also pectoral fin-spines from both the Eocene and 
the Pliocene (Table-case 16). 
Sub-order 2.— Protospondyli. 
So soon as the rays of the dorsal and anal fins had 
become equal in number to their supports, and so soon as the 
upper lobe of the tail had been permanently reduced to an 
insignificant stump, fishes began to advance in the hardening or 
ossification of their internal head-bones and in the acquisition 
of a well-formed back-bone. Each vertebral body originally 
began as four separate pieces surrounding the notochord, 
the upper and lower pairs first uniting into crescents, and 
these two again fusing into a complete ring. The most 
characteristic fishes of the Triassic, Rhsetic, Jurassic, and 
Lower Cretaceous periods were in this condition. They form 
the sub-order Protospondyli (“ first vertebrae ”), and their sole 
survivor at the present day is the “ bow-fin ” or Amia of 
North American lakes and rivers. They are represented in 
the collection by a very extensive series of fine specimens, 
those from the English Lias and Wealden and from the 
Bavarian Lithographic Stone being especially noteworthy. 
The first family is that of the Semionotidse, already repre¬ 
sented by one genus of small fishes, Acentrophorus , in the 
Upper Permian. They are stout-bodied, with a small mouth 
and blunt, often powerfully crushing, teeth. Semionotus and 
Colobodus are Triassic and Rtuetic; Dapedius (Fig. 95) is 
Table-case 
10 . 
Wall-case 
7 . 
Wall-cases 
9 - 14 . 
Table-caseB 
10 - 21 . 
Wall-cases 
9 - 11 . 
Table-cases 
10 , 17 . 
