Manchester Memoirs, Vol. Ix. (1916), No. % 39 



Dipterns platycepJialus in the breaking down of the great 

 shield over the end of the snout and in the loss of that 

 mosaic of small plates which in the older form separates 

 the two main pairs, of plates. Scaumenacia, as shown in 

 Hussakof's drawing which we have been able to corro- 

 borate on a Manchester specimen, differs from Pentlandia 

 macropterus in the narrowness of its anterior region and 

 certain other details. Phaneropleuron differs from Scau- 

 menacia in the loss of the lateral plates of the anterior 

 half of its head and in the fusion of the chief anterior 

 pair into one median bone. Sagenodus differs from 

 Phaneropleuron either, in the loss of this new anterior 

 median element and in the fusion of the only remaining 

 pair of plates which meet in the middle line, or more 

 probably by their separation through a thrusting back of 

 this median element. Ceratodus differs by an extensive 

 fusion of an antero-posterior row of plates. 



This series has every appearance of being a true 

 phyletic sequence illustrating that steady change in a 

 definite direction which is so constant a feature of true 

 phylogeny. It will be noted that these fish are the very 

 ones and in the same order as those which Monsieur Dollo 

 used to illustrate the production of the false diphycercal 

 tail of Ceratodus from the heterocercal tail with separate 

 dorsal fins of Dipterns. Any attempt to read this series 

 in the reverse direction, which, on account of the occur- 

 rence of Uronemus with continuous median fins, and of a 

 Dipterine fish with two small separate dorsals in the 

 Rosebrae Beds at Elgin, might have been attempted if 

 the condition of the fins alone were taken into account is 

 now impossible, for none will be found to suggest that 

 the skull of Ceratodus is more primitive than that of 

 Dipterns. In any case the first known appearances of 

 these fishes in exactly the right order is itself far too 



