187f).] PROF. T. H. HUXLKY ON CERATODUS FORSTKRl. 3.') 



it is itself the dorsal element of the hyoideau arch, attached in its 

 normal position, as its relations to the seventh nerve show. The 

 hvoidean cornua are very strong, and consist of a cartilaginous axis 

 almost completely enslieathed hy bone. The dorsal end of each is 



Fig. 6. 



C 'tratodus fursteri. Inner view of the right hyoidean arch {Hy), with the oper- 

 cular apparatus and part of the suspensorium and of the mandible. 



H.M, hyomandibular cartilage ; Sy, its symplectic process ; Op, operculum ; 

 I.op, interoperculum ; K, R', cartilaginous rays attached to the inner face 

 of the operculum and interoperculum ; VII, exit of the posterior division of 

 the seventh nerve ; k.s, hyosuspensorial hgament (immediately beneath it is 

 the maudibulo-hyoid ligament). 



attached by the strong hyo-suspensorial ligament aforesaid to the 

 suspensorium ; but another very strong round ligament (answering 

 to the mandibulo-hyoid ligament of Menobranchus*) connects the 

 dorsal end of the hyoidean cornu with the angle of the mandible 

 (figs. 4 and 6). 



Thus the hyoidean arch of Ceratodus strikingly resembles that of 

 a Plagiostome on the one hand, and that of an Amphibian on the 

 other. And the small hyomandibular presents a form and connexions 

 which are strikingly similar to those of the suprastapedial cartilage 

 in the Saui-opsidaf, which, on a former occasion, I showed to be the 

 summit of the hyoidean arch and the homologue of the mammalian 

 incus. 



In describing the operculum Dr. Giinther says {I. c, p. 525), 

 " A small movable piece of cartilage is found inside of the articulary 

 groove of the opercle (pi. xxxiv. figs. 2 and 3, k) : it is a rudiment 

 of the prseoperculum." 



Neither the figures referred to nor the account given enable me to 

 be certain that they apply to the cartilaginous structures I am about 



* P. Z. S. 1874, ibid. 



t " On the representatives of the Malleus and Incus of the Mammaha in other 

 Vertebrata," T. Z. S. 1869. 



3* 



