44 Watson & Day, Notes on some Palceosoic Fishes. 



hard shells of which they had to crush, or that they fed 

 on water weeds, which carried sand into the mouth. In 

 any case it is obvious from the dentition that their diet 

 required powerful mastication. It is a general rule, appli- 

 cable not only to fish but to reptiles and mammals, that 

 animals which live by the active pursuit and capture of 

 fish have elongated jaws which provide them with a wide 

 gape and by the oblique insertion of muscles into them a 

 very rapid snapping action. Those forms, however, 

 which have developed a powerful crushing dentition {e.g. 

 Placodus and the Tortoises) have short jaws powerfully 

 built and muscles inserted on to them at right angles. 

 They close their jaws more slowly and then develop 

 pressure on the food. If we compare Dip terns with any 

 Rhipidistian we find that the extreme shortness of the 

 jaw in the former is a very striking feature, which is 

 associated with the fact that its quadrate is inclined for- 

 wards whilst that of the voracious Rhipidistian is always 

 directed backwards, and is always associated with an 

 oblique suspensorium. We therefore suggest that, start- 

 ing from a stage resembling Osteohpis, the Dipnoans, 

 during the gradual concentration of their teeth, resulting in 

 the establishment of a crushing dentition, have gradually 

 shortened their jaws by a swing forwards of the lower end 

 of the quadrate, a process which has squeezed the cheek 

 plates between the circumorbital bones and the operculum 

 until they have vanished entirely from the side of the 

 head and only remain in the triangular notch between 

 the quadrate, the edges of the "supra-temporal''' and 

 " inter-temporals/' and the post-orbital, as illustrated in 



figure. 



Summary. 



In the foregoing paper we have described in detail 

 and given restored figures of the skulls of Holoptychius, 



