ON THK DENTITION OF RHYNCHODUS AND 

 OTHER FOSSIL FISHES. 



Amongst Palaeozoic chimaeroids the complete dentition is 

 known in at least two species of Ptyctodus, two of Rh\ 1 1 

 and one of Palaeomylus. These genera are all included ui the 

 family Ptyctodontidae of the Devonian, and jDresent lor com- 

 parison with recent chimaeroids a single dental plate on each 

 side in the upper jaw, with a corresponding pair biting against 

 the outer side of these (as shown by marks of contact) in the 

 lower jaw. The question therefore arises whether the unique 

 dental plate on each side in the upper jaw of ptyctodonts is 

 homologous with the so-called " premaxillary " of Chimaera, 

 Callorhynchus, Rhinochim^era, etc., or uith the so-called "pala- 

 tine plate" of the latter, or with both premaxillary and palatine 

 taken together. 



This question appears to be answered conclusively m the case 

 of Ptyctodus, from which the modern type of dentition is derived 

 by pushing the low and elongate upper dental plate further back 

 in the mouth, and introducing a "premaxillary " or "vomerine 

 tooth in front of it. In all cases the lower dental plate is 

 vertically deeper than the upper, and rises into a i)i-ominent 

 beak anteriorly. It is also characterized by having a desceiuhng 

 process at the symphysis, which is more accentuated m the 

 fossil than in recent forms. This process bears a triangular 

 groove or excavation on its inner face, the roughened surface of 

 which indicates that it was occupied by cartilage, since there 

 was no sutural union at the symphysis. That this was the case 

 is self-evident, for the anterior Leaks could not have clo.sed 

 outside the upper dental plates when the mouth was shut unless 

 the lower ones were mutually separated by a slight interval. 



In a recent communication by Jsekel,^ it is stated that "von 



1903, pp. 383-393- 



