82 SECTIONAL ADDRESSES. 
Now let us leave for a time the origin of species and consider the 
origin and evolution of a sub-class, the Neopterygian Fishes, the group 
that includes the great majority of living fishes and of which the most 
primitive living representatives are Lepidosteus and Amia. The earliest 
Neopterygians were the Semionotide, which began in the Upper Permian, 
and the only known fishes that can have given rise to them are the Palzo- 
niscids, which flourished from Devonian to Permian times and had fins 
essentially similar in structure to those of a sturgeon. The transforma- 
tion of a Palzoniscid into a Semionotid can be interpreted as the trans- 
formation of a strong-swimming fish that captured other fishes into a slow- 
swimming fish that fed at the bottom on small molluscs and crustaceans. 
The reduction of the upturned end of the tail was related to a lesser 
speed in swimming ; the decrease in number and spacing out of the rays 
of the dorsal and anal fins, until they were equal in number to their skeletal 
supports and each acquired a definite articulation with its own basal bone, 
made these fins less suitable for cleaving the water in swift motion, but 
better fitted to perform the delicate movements required of the fins of a 
fish that swims about slowly. The change from a wide mouth, with 
strong, sharply pointed teeth, set well apart, to a small mouth, with small, 
blunt teeth, set close together, was related to the change in food. In 
connection with the small size of the mouth the suspensorium became 
directed forwards and the preoperculum acquired a long lower limb, 
and below this lower limb appeared a new bone, the interoperculum, 
which looks like an anterior outgrowth of the suboperculum that segmented 
off in order to preserve freedom of movement. The lower jaw of the 
Semionotide was short and broad, probably used for crushing shells ; 
in relation to this, another new bone, the symplectic, was developed to 
articulate with its hinder end external to the quadrate articulation. 
The characters diagnostic of the sub-class Neopterygu, the abbreviate 
heterocercal or homocercal caudal fin, the dorsal and anal rays equal in 
number to and directly articulated with their skeletal supports, the presence 
of an interoperculum and of a symplectic, were all adaptive when first 
acquired and were related to a change in food and in feeding habits. 
The Semionotide gave rise to a number of distinct families, two of 
which are of special interest. The Eugnathide were active, predacious 
fishes, resembling the Paleoniscide in the size of the mouth, the denti- 
tion, and the form of the fins. But although the dorsal rays have m- 
creased in number and become concentrated, the dorsal fin is quite unlike 
that of the Palsoniscide, for the skeletal supports have imereased in 
number with the rays; similarly the forked caudal fin differs in that its 
upper lobe is formed by the outgrowth of fin-rays and does not include the 
upturned end of the tail. The resemblances between the Paleoniscide 
and the Eugnathide are adaptive ; the differences are not adaptive, but 
historical, due to the Semionotid ancestry of the Eugnathide. 
The Jurassic Pholidophoride, also derived from the Semionotide, 
were extremely like herrings in shape, in the form and position of the fins, 
and in the rather small and feebly toothed mouth ; doubtless they were 
plankton-feeders. In correlation with the small size of the teeth the jaws 
were slightly built and the symplectic articulation with the lower jaw was 
lost ; the presence of this bone became a historical character. Towards 
the end of the Jurassic the Pholidophoride gave rise to a group of larger 
