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Show 1894.] HYOIP ARCH OF OERATOPUS. 635 and on comparing his figs. 3 and 4 it will be seen that the hyoidean nerve passes in front of the hyomandibular in Silurus and behind the suspensorium of Ceratodus, which in his figure he separates from the palatine cartilage by a firm line and designates the " hyomandibular." Nerves, from their early distribution, may safely be employed as tests of homology of parts, and the spiracle, being as it is a visceral cleft, has claims of equal importance, so that Pollard's deduction, drawn from his conclusion of the non-homology of the hyomandibular of Teleostei and Elasmobranchii, that the spiracle and the hyoidean nerve are not constant in their relations to the hyomandibular, tends to falsify the premises. It is not, however, on this account to be assumed that the hyomandibular of Teleostei is here considered homologous with that of the Elasmobranchii, for, as first suggested by Parker, later researches may subsequently show their want of correspondence. Seeing that accessory nodules of cartilage are so common as fringes to the opercular and interopercular bones and on the cranial ribs (as also in the Sturgeon), it might certainly be suspected that the " hyomandibular " of Ceratodus, lying as it does on the inner side of the operculum, belonged to this category, but its relation to the seventh nerve and to the ceratohyal, and its position between the quadrate cartilage and the first branchial arch, all point to its being a constituent of the hyoid arch. Gadow (3) writes (p. 458) :-" The outer surface of this hyomandibular remnant is loosely connected with the small cartilaginous operculum, which w e know to be the result by fusion of the branchiostegal rays, carried by the hyomandibula." The name opercular has hitherto been applied in Ceratodus to the large bone of the dermal series, situated behind the pre-opercular aud fringed with accessory cartilages, and to re-apply the name, without apology or argument, to a small cartilage must necessarily lead to confusion. There is no synovial articulation, in Ceratodus, between the hyomandibular and the opercular, on the inner surface of which it lies, such as occurs in the Teleosteans and the bony Ganoids. The anterior edge of the hyomandibular is united with the skull just where the cranium proper passes into the suspensorium ; and the distance between this spot and the auditory capsule, in the vicinity of which the hyomandibular of fishes usually articulates, marks the extent of the reduction which the upper part of the hyoid arch has undergone. This hyomandibular cartilage was apparently overlooked by Giinther (4), who figures (pi. xxxiv. fig. 3, r) a tubercle for the suspension and articulation of the hyoid arch. A comparison of the original specimen with this figure, which by the kindness of Dr. Giinther I was allowed to make, shows that this is, as he states, a protuberance of the suspensorium for the attachment of the ligament suspending the ceratohyal; and |
