OCR Text |
Show 1876.] PROF. T. H. HUXLEY ON LEPIDOSIREN. 181 latter are the edges of the fold which overhang the mandible on each side when the parts are undisturbed. They occupy the maxillary region of the head, and may be termed the maxillary lips. The median portion of the anterior contour (a b) does not belong to the lips at all, but corresponds, as is easily seen in a section of the skull, with the anterior extremity of the chondrocranium and the ventral edges of the nasal capsules. The line a b is, in fact, the anterior contour of the head ; and the pigment-cells of the dorsal integument are here continued onto the ventral face. After a very short distance, however, the integument terminates in a slight, transverse, papillose ridge, which forms the anterior boundary of a shallow median depression ; on each side of the hinder part of this depression is one of the vomerine teeth (v.t). Under view of the fore part of the roof of the mouth and underside of the head - of a young specimen of Lepidosiren anneclens ( X 3). a b, anterior margin of the head; ac, bd, right and left maxillary lips: a.n, anterior nasal apertures; p.n, posterior nasal apertures; u.l, praemaxillary lip; v.t, vomerine teeth; p.t, palatine teeth. The anterior nares (a.n) He immediately behind the anterior contour of the head ; aud their more prominent posterior lips lie in front of a transverse line drawn through the edge of the papillose ridge. It is obvious that the area included between the anterior contour of the head in front, the median fossa behind, and the anterior nares at the sides, corresponds with the region occupied by the naso-frontal process in the vertebrate embryo and with the homologous large, shield-shaped, naso-frontal integumentary plate in the Rays and Dogfishes. The papillose ridge (u.l) is therefore the middle or premaxillary portion of the upper lip ; and if this be so, it follows that the anterior nares in Lepidosiren are placed as in Chimaeroids and Plagiostomi, on the under aspect of the head and outside the mouth. I may add that Lepidosiren has two upper labial cartilages-one fibrocartilaginous immediately behind the anterior nasal aperture, and the other behind the posterior nasal aperture. These answer to the upper labial cartilages in Chimara and Cestracion. |