82 CATALOG or FOSSIL PISHES IN THE MUSEUM 



type, which shows a similar symphyseal region and symphyseal 

 teeth. In the type the blade or inserted portion of the mandible is 

 missing, but a small projection of bone on the lower surface near the 

 posterior end of the specimen apparently represents the blade portion 

 and shows that it was present. 



The most remarkable character of the mandible is, of course, the 

 symphyseal region, with the 7 symphyseal teeth. These are so 

 clearly shown (PI. 20, figs. la, ib) as to leave no question as to their 

 number, size, form or direction. The symphyseal region has much 

 resemblance to the denticled symphysis of Coccosteus, as shown for the 

 latter in the few specimens in which it is preserved; but the symphy- 

 seal teeth in Perissognathus being so much larger are far better shown 

 than in any specimen of Coccosteus. 



There has been considerable discussion of the meaning of the sym- 

 physeal denticles in arthrodiran mandibles, and reference may be made 

 to the detailed study of the subject by Hussakof, in 1906.^^ The 

 present mandible throws some light on the problem. In the first 

 place, the upper part of the symphyseal area shows lines of wear 

 parallel to the axis of the beak, produced by the upper tooth. Had 

 the two halves of the lower jaw been united at the symphysis it is 

 inconceivable how an upper dental plate could have played against 

 the symphyseal area on each side, and the two halves of the lower jaw 

 still remain united. Secondly, the direction of the sjonphyseal teeth, 

 which do not stand at right angles to the inner vertical margin of the 

 s)miphyseal area but slant upward at an angle of about 45°, shows that 

 these teeth could not have interdigitated; it is a mechanical impos- 

 sibility for the s)miphyseal teeth to interlock unless they stood at right 

 angles to the vertical margin of the symphysis. 



It may be mentioned that the mandible of Perissognathus is also 

 somewhat suggestive of a P alceomylus dentdX plate — at any rate, in its 

 present form, with the hinder blade lacking. It thus serves like 

 several other forms that might be mentioned to connect the Ptycto- 

 dontidse with the Arthrodira, and so lends support to the view set 

 forth by Dollo, in 1907, in his paper, "Les Ptyctodontes sont des 

 Arthroderes."36 



The following specimen also belongs to Perissognathus aduncus. 

 It is from the same formation and locality as the type — Conodont 

 bed. Eighteen Mile Creek — and was collected by W. L. Bryant. 



'^Studies on the Arthrodira. Mem. Am. Mus. Nat. Hisl., ix, ii8, 121, 122, et passim, igo6. 

 ^ Bull. Soc. Beige de Geol., xxi, 12 pp., pi. ii. 



