278 Mr. A. S. Woodward on the Myriacantliidge. 



suggests itself as worthy of consideration ; but, since the 

 type specimen was entirely misinterpreted by Egerton, and as 

 a new specimen in the British Museum (no. P. 6095) is now 

 available for study, it is necessary, in the first place, briefly 

 to recapitulate the principal features of this singular group of 

 teeth. 



As pointed out by Dr. von Zittel *, the mandible of the 

 type species, Prognathodus Guentheri^ was mistaken by Eger- 

 ton for the upper jaw, while the upper dentition was ascribed 

 to the lower; and thus arises the necessity for a complete 

 revision of tlie subject. The mandibular cartilage precisely 

 resembles that of modern Chimferoids in exhibiting no sym- 

 physial suture ; and there is likewise a pair of small inferior 

 labials. The right and left lower dental plates (m in Eger- 

 ton's figure) are of the form ordinarily observed in Chimaj- 

 roid fishes and meet in a narrow facet at the symphysis ; 

 but in advance of these is a median, bilaterally-symmetrical, 

 incisor-like element [p^ Egerton), also without doubt to be 

 assigned to the mandibular dentition, and hitherto unparalleled 

 in the order or subclass to which the fish belongs. The pala- 

 tine plates (1, Egerton) are large^ expanded, and thin, elon- 

 gated antero-posteriorly, and irregularly triangular in shape ; 

 the outer margin of each is somewhat thickened, while both 

 the outer and anterior margins are sharply deflected ; the 

 inner and posterior margins are thin edges, and there is no 

 appearance of the close apposition of the right and left plates 

 in the median line. The vomerine dental plates (2, Egerton) 

 are, as usual, much smaller than the palatines and in direct 

 contact both with the latter and with each other ; they are 

 triangular in form and comparatively thin, and owe their 

 robust aspect to the sharp deflexion of the margins. Still 

 more anteriorly, in the known specimens, is a pair of small 

 quadrangular, transversely ridged teeth (3, Egerton), which 

 may be either independent elements or merely the accidentally 

 detached front processes of the vomerine dental plates. 



Thus interpreted, it is obvious that the dentition of Progna- 

 thodus Guentheri repeats the two most peculiar features noted 

 above in the new specimen of '^Ischgodus orthorhinus.''^ 

 There is the same incisor-like median tooth, referable to the 

 lower jaw ; and the front portion of each vomerine dental 

 plate is transversely ridged in the same manner. Moreover, 

 ^^Prognathodus " possesses tuberculated dermal plates upon 

 the head, resembling the example alluded to above in the 

 second specimen of ^^Ischyodus orthorhinus ; " and a careful 

 comparison of all recognizable characters appears to the 

 * Op. cif. vol. iii. p. 115. 



