776 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [30 J 



without doubt, a retrogressive process is presented us, as in the snakes; 

 and the alternative proposition, that in these forms a middle ear has 

 not yet developed, is untenable. In fact it would be incomprehensible, 

 if the closed foremost gill cleft of the higher Amphibia were to reopen 

 itself and re-enlist its functions in connection with the auditory ap- 

 paratus. Equally unintelligible would be the occurrence of the col- 

 umella in XJrodela — a part of the skeleton whose origin is closely asso- 

 ciated with the development of the middle ear, and if it existed by 

 itself its need could not be understood, inasmuch as no function for it 

 could be discovered. 



The question now remains whether the apparatuses we have just de- 

 scribed for the bony fishes and the Selachians originated entirely inde- 

 pendently of each other, as appeared at the first glance, or whether 

 there are not organs somewhere in existence which constitute the con- 

 necting links between them, and allow a genetic connection of these 

 apparently entirely different formations to be entertained. 



A direct comparison of the apparatuses in the Selachians with the 

 Teleosteans leads to an unsafe result, inasmuch as the topographical 

 appearances on the skulls of these forms are entirely different, and as 

 a natural consequence the various relations of the parts cannot be com- 

 pared with each other in detail ; therefore it only remains for us to look 

 about us for the intermediate forms and through them attempt the 

 solution of the question. Such an absolutely intermediate form — of 

 course only for the purpose mentioned — is Polypterus. While the cra- 

 nium of this Sauroid, and particularly its maxillary apparatus and gill 

 apparatus, very closely approach the Teleostean type, the Polypterus 

 during life possesses a well-developed hyomandibular cleft, and in this 

 respect reminds us of the Selachians. The inner, capacious opening of 

 this cleft lies in the gill cavity ; it is bounded mesially by the epi- 

 branchiae of the first gill arch, posteriorly by the anterior margin of the 

 hyomandibular, and laterally by the bones of the palatal arch. This 

 wide hyomandibular cleft- takes an upward direction, lying close to the 

 labyrinth region of the skull, to make its exit at the upper and lateral 

 margin of the cranium in a slit-like opening, that is covered by two 

 small dermal bones, which act like valves. In Polypterus the conduction 

 of the sound-waves to the labyrinth can scarcely take place through 

 the outer opening, closed as it is by the small dermal bones just referred 

 to, so we must believe that the sound-waves enter at the inner and least 

 difficult opening, as this does not open into the buccal cavity — as in 

 the Selachians — but into the gill cavity, which is in complete communi- 

 cation with the outer medium. 



A comparison of the hyomandibular cleft in Polypterus with the blind 

 apical recess lying close to the labyrinth region in bony fishes pla jes it 

 beyond all doubt that they are homologous structures, and that this 

 recess of the gill cavity, which was alluded to when speaking of the 

 Teleosteans, is nothing more than the hyomandibular cleft which has 



