1894.] HYOID ARCH OF CERATODUS. 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 dis- 

 tribution, 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 corre- 

 spondence. 



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 hyo- 

 mandibular remnant is loosely connected with the small carti- 

 laginous operculum, which we 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 



