26 WATSON & DAY, Notes on some Palceozoic Fishes. 



have been of the great length which would have been 

 necessary if it were to have given rise by a basal concen- 

 tration to the elongated "archipterygiai" fin of Holoptychms 

 and we are therefore driven to the conclusion that the 

 elongated type of " archipterygiai " fin has arisen from a 

 short fin similar to that of Tristichopterus. 



The facts which we have brought forward throw doubt 

 on the usefulness of the current divisions of the Rhipidistia. 

 Fish so obviously closely allied as Tristichopterus and 

 Eusthenopteron may differ considerably in their skull 

 structure, whilst fish with so exactly similar heads as 

 Gyroptychins and Osteolepis may have a family separation. 

 The distinction between the families Rhizodontidae and 

 Osteolepidae has been drawn essentially on the possession 

 of " cycloid " scales in the one and rhombic in the other 

 group. In other families of fishes, the Palseoniscidae, 

 Semionotidae and Dapedidae for example such difference 

 is not regarded as of family value, and known forms of 

 Rhipidistia exhibit a transition from one type to the other. 

 It is obvious that the condition of the caudal fin cannot 

 be used for family division amongst these fishes, nor can 

 we find any features in the skull which are more useful. 



When we come to the Holoptychidae we seem to have 

 in the elongated pectoral fins a more satisfactory basis 

 of separation, but the quite elongated pectoral fins of 

 Glyptopomus, the discovery of which led Mr. Goodrich to 

 place that fislf in a separate family the Glyptopomidae, 

 are really intermediate in form between the fins of 

 Rhisodopsis and those of Holoptychius. It appears to us 

 that only two courses are possible, to form a large series 

 of families each containing fish which are obviously closely 

 allied, which families would be at present impossible of 

 concise definition, or a recognition of only a single family. 

 As the institution of many almost indefinable families 



