28 MR. W. K. PARKER ON THE STRUCTURE AND 



and the long, 2^>'6cuciuus, tllamentoiis gills of the embryo Selachian, are, like gills 

 generally, of hypohkustic origin. 



The vertical longitudinal section (fig. 5) may be now described. 



The large upper labial (below c.tr.) is seen in section; it is the "anterior dorsal 

 cartilage " of J. Muller : below, the lower labial is seen, " where it joins its fellow to 

 form the imperfect "annular cartilage" (see also fig. 4, l.l., mk.). 



Meckel's cartilage is seen behind this in the section ; it lies in the side of the 

 mouth, and its main function is to serve as a suspensorium to the " annular cartilage " 

 (the con-esponding half of the imperfect ring) (see also Plate 2, fig. 6, l.l, mk). 



The raised cushion behind these cartilages is the rudimentary tongue (fig. 4, tg.) ; 

 in the back of this the basal cartilages are placed. 



In most things I agree with Professor Huxley as to his harmony of the Lamprey's 

 and the Tadpole's mouth. There is some reason to suppose that the antero-inferior 

 median cartilage belongs to the mandibular arch. The basi-hyal of Fishes is always 

 a "glosso-hyal;" it projects forwards beyond the arch to which it belongs, and so does 

 the first basi-branchial. 



Any paired plate of cartilage lying in the same plane, but projecting outwards and 

 backwards from the basi-branchial bar (or bars) would be "hypo-visceral;" any rods 

 attached above these and lying in the lower half of the sides of the pharynx would be 

 " cerato-viscerals." 



In the Lamprey (Huxley, op. cif., plate 17, fig. 1) this anterior median cartilage, 

 below, is outside the luigual rod, and is probably a basi-mandibular ; and the great 

 lingual cartilage (k.) is an undivided piece, with neither osseous nor fibrous segmenta- 

 tions, and is evidently an intra-visceixd element. 



In the Tadpole (Plate 2, fig. 8, b.hi/.) the anterior basal piece does not chondrify 

 until metamorphosis takes place ; it is a thick, naiTow, conjugating band, composed 

 of large cells of simple cartilage. 



But the basi-branchial of the Tadpole (b.br.) is an oval mass of cartilage, with a 

 hinder bud that grows downward beneath the large hypo -branchial plates {h.br.) ; this 

 bud does not become a separate second basi-branchial. These two semi-distinct 

 rudiments do not represent more than the end of the long lingual cartilage of the 

 Lamprey ; aiid if that fish had paired cartilaginous plates lying on the same plane as 

 the lingual cartilage and attached to its sides, behind, they would be hypo-branchial 

 plates, for they would belong to the inner category of cartilages, and not to those 

 forming the branchial basket-work. 



In the Tadpole the heart (fig. 5, h.) is roofed over by the " hypo-branchial '' plates, 

 and the edge of the "inferior velum" (fig. 4, l.v.) is seen completing this roof and 

 also covering the inner branchial openings in front. 



Where the right and left vela unite at the mid-line, there they are directly in front 

 of the larynx (Ix.) ; afterwai-ds, the hypo-branchials become the " thyro-hyals." 



* The iiuKT liici; oT thu lower lahiiil is lettered luk., by raistiike, in Plato 1, tig. b. 



