1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 25 



Thus the anterior nares can in no sense be said to open into the 

 cavity of the mouth, inasmuch as they lie outside the praimaxillary 

 portion of the upper lip, and are not enclosed by the maxillary por- 

 tion of that lip. They are not even placed between the upper and 

 the lower lips, inasmuch as the vaulted flap, on the underside of which 

 they He, is not the upper lip, but the anterior part of the head. 



In Lepiclosiren, the anterior nares are closer to the anterior margin 

 of the head than in Ceratodus, and the prsemaxillary lip is repre- 

 sented only by a papillose ridge, in which the integument of the 

 underside of the head, between the anterior nares, terminates pos- 

 teriorly. Otherwise the disposition of the nostrils is quite as in 

 Ceratodus ; and when the mouth is shut, the nostrils open on the 

 underside of the head, in front of it and of the rudimentary prse- 

 maxillary portion of the upper lip. 



The disposition of the nasal apertures in the Dipnoi is essentially 

 Selachian *. In the common Dogfish {ScyUium), for example, the 

 anterior contour of the head answers to the anterior contour of the 

 head of Ceratodus. The mandibular and maxillary lips are similarly 

 disposed ; and the external nares are placed on the sides of the head 

 in a similar position. But the praemaxillary part of the upper hp 

 is much larger and more prominent; audits outer edges (septal alse), 

 instead of being continued into the maxillary lip, to form the floor of 

 the nasal passage, are separated from it by a fissure, which commu- 

 nicates with the nasal cavity. This fissure is overlapped by the 

 septal alse ; and thus an incomplete nasal passage, which opens pos- 

 teriorly into the canity of the mouth, is constituted. 



Still more instructive is the comparison of the nasal passages of 

 Ceratodus with those of Cestracion and Chimcera. In Cestracion, 

 the external nostrils lie just outside the mouth, the lower lip coming 

 into contact with the maxillary and praemaxillary portions of the 

 upper when the mouth is shut. ' The anterior end of the maxillary 

 lip is folded in, and passes into the external part of the ala nasi, which 

 has a thickened edge, and ends in a continuation of the free fold of 

 the lip. The inner ala nasi is the outer part of the internasal or prse- 

 maxillary part of the upper lip. It also ends in a free edge, which is 

 rolled inwards. The septal ala and the maxillary ala do not unite ; 

 but a groove is left between their convex edges, which answers to 

 part of the groove which leads from the nose into the mouth in Dog- 

 fish and other Plagiostomes. But the greater part of this groove is 

 represented by a canal formed by the convoluted septal ala, which is 

 open on its dorsal aspect, and communicates, in front, with the cavity 

 of the olfactory sac. Behind, the free edge of the septal ala has a 

 curious frmge ; and when the mouth is shut, this fringe overlaps the 

 edge of the mandible. The free edge of the septal ala bounds a 

 large opening, the posterior nostril, which is situated, as in Cera- 

 todus, at the point of junction between the vomerine and the palatine 

 teeth. Consequently, when the mouth is shut, there is a free passage 

 for water through this incompletely closed nasal canal. ° 



» See the excellent observations of Gegenbaur, ' Kopfskelet der Selachier ' 

 p. 224 et seq. ' 



