22 WATSON & DAY, Notes on some Palceozoic Fishes. 



reduction in the number of elements in the head. 

 Seymouria, undoubtedly the most primitive of known 

 Reptiles, has in its " intertemporals " bones which occur 

 in no other reptiles, and all the later reptiles, birds, and 

 mammals show the gradual loss of elements, which seems 

 to have taken place, in the majority of cases at any rate, 

 by a gradual reduction in size, followed by disappearance, 

 and not by a fusion of neighbouring bones. One would 

 expect that in fish also reduction of number of parts 

 would be a feature in evolution and that it would be 

 accompanied by the actual disappearance of bones and 

 not by their fusion with neighbouring elements. From 

 the restorations given in this paper it is perhaps possible 

 to gain some idea of the truth of this statement, which 

 may perhaps be done most easily by comparing 

 HoloptycJiius with Glyptopomns. The general homologies 

 of the bones are quite clear. Two small bones, the 

 supra-orbital ossicle and that lying between the post- 

 orbital and the frontal, which occur in Glyplopotnus are 

 not found in Holopiy chins or indeed in any other 

 Palseozoic fish. The interesting region however is that 

 which lies on the cheek behind the post-orbital and jugal. 

 This region is in the one case covered by three bones, in 

 the other by four or five. Two of these the quadrato- 

 jugal and the pre -operculum are obviously homologous 

 and the only question left is therefore whether the two 

 or three plates in HoloptycJiius have arisen by the 

 fragmentation of the single large plate in Glyptopomns, 

 whether they have fused to form the single plate of 

 Glyptopomns, or whether the squamosal of Holoptychius 

 has expanded crushing the others out of existence. 

 Consideration of the actual form of the posterior border 

 of the squamosal of Glyptopomns which provides a step in 

 which the anterior corner of the pre-operculum is embayed 



