266 



combiaation of the booes of Teleosts (No. 33, p. 405). If, however, 

 a fresh head of Polypterus be examined it will be seen that the outer 

 dermal flap, or fold, of the upper lip of the fish, which lies wholly 

 lateral to the so-called maxillary and its surrounding tissues (No. 32, 

 Figs. 34 and 35), occupies a position that corresponds strikingly with 

 that of the dermal fold that encloses the maxillary bone in Amia and 

 in many Teleosts. Is then this maxillary labial fold of Polypterus, 

 which contains no bone, the homologue of the fold of Amia, which 

 encloses the maxillary bone? And if it is, with what bone in the 

 Teleostean skull is the tooth - bearing part of the maxillary bone of 

 Polypterus homologous? 



The maxillary labial fold of Polypterus, and also the mandibular 

 labial fold, have each the appearance of having been formed by the folding 

 back upon itself of the produced free edge of the upper or lower lip, 

 respectively, of the fish; and Huxley (No. 21, p. 24) so describes the 

 formation of the strictly homologous mandibular labial fold of Cera- 

 todus. The free upper and lower edges of the maxillary and mandi- 

 bular folds, respectively, would, under this supposition, represent the 

 original oral edges of their respective lips. A little consideration of 

 the lips of Polypterus will show that this is wholly improbable, and 

 that the labial folds of the fish must have been formed by the deve- 

 lopment of a furrow that has cut gradually deeper and deeper into 

 each lip, starting upon its external surface and cutting downward or 

 upward as the case may be, toward the oral edge of the lip. The 

 maxillary fold of Amia is certainly formed by the gradual development 

 of such a furrow, the furrow being the supramaxillary furrow of my 

 descriptions (No. 1). The mandibular fold of Amia is also quite un- 

 questionably formed in this same way, but its developnient is not com- 

 pletely shown in my material. In Ceratodus it is also apparently so 

 formed, as an examination of Semon's (No. 39) figures will show. 



The maxillary fold of Polypterus begins, anteriorly, at the base 

 of the anterior nasal tube, and extends backward to the postero-inferior 

 corner of the so-called labial cartilage, the hind end of the fold being 

 attached along the entire length of the ventral edge of the labial. The 

 mandibular fold begins, anteriorly, at a point on the mandible corre- 

 sponding to, and approximately opposite, the anterior end of the 

 maxillary fold, and like the latter fold it ends posteriorly at the 

 postero-inferior corner of the labial, its hind edge beir)g also attached 

 along the ventral edge of that element. Internal to each of these folds, 

 and at their bases, there is another small fold, which begins posterior 



