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MR. C. TATE REGAN ON THE 



The fourth Order, the Isospondyli, is distinguished from the 

 Halecostomi by the liomocercal structure of the cavidal fin ; it is 

 followed by the Orders Ostariophysi, Apodes, etc. 



The relationships of the families dealt with above may be 

 expi-essed diagrammatically as follows : — 



Lepidosteidje 

 Aspidorhynchidse. 



Elopidpe. 



/ 

 Leptolepidse. / 



Pachycormidse 



Amiidse 



Pycnodontidse. 



Oligopleuridse. 



Semionotidsc. 



Palseoniscidse, 



General Considerations. 



In dealing with the origin and evolution of the lower Neoptery- 

 gian Fishes I have tried to emphasize the conclusion at which 

 I have arrived, that their diflferentiation has been adaptative, 

 changes of structure being especially related to the nature of 

 the food and the method of procuring it. 



This is well illustrated by the following example : — The Semi- 

 onotidse, probably slow-swimming fishes feeding on molluscs, 

 crustaceans, etc., appear to have been derived from the Palseo- 

 niscidse and have themselves given rise to the Eugnathidae, 



