238 report— 1846. 



called ' cerato-branchials.' It is with these elements of the "branchial arches 

 in fishes and perennibranchiate batrachians that we are chiefly concerned 

 in tracing the homology of the hyoid apparatus in the air-breathing verte- 

 brates. With regard to the branchial and pharyngeal arches, which attain 

 their full development only in the class of fishes, I regard them as appertain- 

 ing to the system of the splanchno-skeleton, or to that category of bones to 

 which the heart-bone of the ruminants and the hard jaw -like pieces support- 

 ing the teeth of the stomach of the lobster belong. The branchial arches 

 are sometimes cartilaginous when the true endo-skeleton is ossified : they are 

 never ossified in the perennibranchiate batrachians, and are the first to dis- 

 appear in the larvas of the caducibranchiate species ; and both their place 

 and mode of attachment to the skull demonstrate that they have no essential 

 homological relation to its endo skeletal segments. 



The hyoid arch or apparatus retains most resemblance to that of fishes in 

 the Siren lacertina ; the basihyal is simplified into a single osseous spatu- 

 late piece, with the bowl of the spoon anterior, and supporting a broad and 

 flat semicircular glossohyal. A strong and thick ceratohyal is articulated 

 by means of a small cartilage to the side of the expanded part of the basi- 

 hyal, and a cartilaginous epihyal arches backwards from its upper end. A 

 cartilaginous urohyal extends from the hind end of the basihyal, and ex- 

 pands into a radiated disc, which supports the membranous trachea and the 

 simple glottis. One pair of bony ' hypobranchials ' is articulated to the 

 basi-uro-hyal joint and a second pair to the sides of the urohyal : and to the 

 upper and outer ends of these are attached four pairs of cartilaginous ' cerato- 

 branchials.' The fimbriated branchiae are attached to the three anterior 

 ceratobranchials. 



In the proteus the urohyal is absent, and it is not again developed in any 

 batrachian. The long subcylindrical basihyal supports a subcircular carti- 

 laginous discoid glossohyal, and at the angle of union the bony ceratohyals 

 are sent olF. A pair of hypobranchials diverge from the end of the basihyal ; 

 to which a second small pair of basibranchials are loosely connected by an 

 aponeurosis. These support three ceratobranchials on each side, which are 

 bony. 



In the newts there is neither a glossohyal nor urohyal, or but a rudiment 

 of the latter, to each side of which are articulated two hypobranchials, whose 

 distal ends converge on each side to support a single cartilaginous gill-less 

 rudiment of a ceratobranchial. The special homologies of all those parts of 

 the complex hyoid, rendered more complex by the retention of part of the 

 branchial skeleton, are clearly demonstrated by pursuing the metamorphoses 

 of the hyo-branchial skeleton in the larvae of the anourous batrachians. In 

 the full-gilled tadpole a short and simple basihyal supports laterally two 

 thick and strong ceratohyals, and posteriorly two short and broad hypo- 

 branchials, to which four ceratobranchials are attached : all the parts are 

 cartilaginous. The type of this stage is retained in the siren, with the histo- 

 logical progress to bone in the hyoid and hypo-branchial pieces. The second 

 well-marked stage in the tadpole shows an extension of the external and 

 posterior angles of the hypobranchials, with progressive absorption of the 

 cartilaginous ceratobranchials. The growth and divergence of the posterior 

 angles of the hypobranchials refer to the development of the larynx, now 

 commencing, which part they are destined to support. That period may be 

 described as the third stage at which the ceratobranchials have disappeared, 

 and the posterior angles of the hypobranchials increase in length and assume 

 the character of posterior cornua of the os hyoides. The last and adult 

 stage shows the ossification of the elongated angles of the hypobranchials, 



