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Show 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 praemaxillary portion of the upper lip, and are not enclosed by the maxillary portion 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 lie, is not the upper lip, but the anterior part of the head. In Lepidosiren, the anterior nares are closer to the anterior margin of the head than in Ceratodus, and the praemaxillary lip is represented only by a papillose ridge, in which the integument of the underside of the head, between the anterior nares, terminates posteriorly. 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 praemaxillary portion of the upper lip. The disposition of the nasal apertures in the Dipnoi is essentially Selachian *. In the common Dogfish (Scyllium), 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 lip is much larger and more prominent; and its outer edges (septal alae), instead of being continued into the maxillary lip, to form the floor of the nasal passage, are separated from it by a fissure, which communicates with the nasal cavity. This fissure is overlapped by the septal alae; and thus an incomplete nasal passage, which opens posteriorly into the cavity 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 praemaxillary 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 Dogfish 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 fringe ; 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 Ceratodus, 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. |