[29] THE OSTEOLOGY OF AMIA CALVA. 775 



unfortunately his observations appear to have entirely passed into 

 oblivion, at least 1 have not come across a single allusion to them in 

 the writings of the more recent authors. The sound- conducting appa- 

 ratus in the Selachians is the hyomandibular cleft. This starts, as we 

 know, with a wide opening in the buccal cavity in a position nearer the 

 median plane than the opening of the first gill cleft, and close to it, 

 and then courses upwards between the hyomandibular and the palato- 

 quadratum, making its exit either in an opening, the aforesaid hyoman- 

 dibular cleft, behind and above the eye, or ending blindly beneath the 

 skin. During its course this canal lies close to the labyrinth region, 

 and in individual cases it even presents special blind diverticles, which 

 adhere closely to it. This is the point in Selachians where the labyrinth 

 is nearest the surrounding medium, and through this channel the sound- 

 waves must reach it the least diminished in intensity. That they may 

 be conducted, too, from the surface of the head, is by no means to be 

 set aside — such general transmission, to a limited extent even taking 

 place in man through parts of the skeleton of the head — yet the idea 

 of such a conduction in the Selachians, if the parts concerned are investi- 

 gated according to physical principles, must be utterly abandoned, 

 when we come to compare this with the parfe played as a conductor by 

 the hyomandibular cleft [speitzloch canal]. The sound-waves to only 

 a limited degree can enter the hyomandibular cleft from the cavity of 

 the mouth, and will at least in cases where there is a wide, open, 

 external cleft existing, find their entrance through it. 



The fact that the hyomandibular cleft of the Selachians being 

 homologous with the tyinpanic cavity and the canals in the higher 

 vertebrates, and exercising a similar function, is certainly very remark- 

 able. This demonstration effects the removal of one difficulty, and 

 that is the belief that the tympanic cavity and the canals first originated 

 among the air-breathing vertebrates. In fact it was scarcely at all 

 understood how for this purpose, a gill cleft, whose very existence de- 

 pends upon its being constantly in water, could continue to exercise its 

 true function, and still to some extent be subservient to the organ of 

 hearing. This difficulty is completely set aside by the discovery that 

 the sound-conducting function of the anterior gill cleft is not a new 

 acquisition in land vertebrates, but that it also existed in their ancestors 

 living in the water; and with these the reason [ursdchliche moment] 

 for this is also furnished, why this gill cleft could still survive, retain- 

 ing its integrity to the very last and in the most advanced vertebrates 

 in the scale of development, while the other gill clefts, originally pro- 

 vided with respiratorial functions, have disappeared without leaving a 

 trace, having commenced in the Dipnoi and Amphibia with the devel- 

 opment of a new respiratory organ. 



After what we have just demonstrated, the fact that the Urodela and 

 several of the Anura possess no tympanic cavities or Eustachian tubes, 

 is to be differently construed from what it has been heretofore. Here, 



