1894.] 



NATURAL SCIENCES OF PHILADELPHIA. 



49 



outside the membrana tynipaui which, when approximated, close the 



entrance thereto. In the cro- 

 codilia each tympanum (Fig. 

 12, T) is not only provided 

 with its own lateral Eustachian 

 tube proper, LE, but the two 

 tympana communicate directly 

 with each other superiorly by 

 means of a passage traversing 

 the supra occipital bone, S, 

 and inferiorly by anterior 



Fig. 12. Diagram of tympauic canals of lateral passages, C, descending 

 alligator. from the floor of the tympanum 



into the anterior branch. A, of median Eustachian tube, ME, situated 

 just behind the posterior nares and indirectly by posterior lateral pas- 

 sages, B, given off on either side from the lateral Eustachian tubes proper, 

 LE, and terminating in the posterior branch, P, of the median Eus- 

 tachian tube, ME. Further passages extend from each tympanum, 

 Q, through the quadrate, thence by a membranous tube into the 

 articulare of the lower jaw.'*^ 



In the turtle the most posterior of the two lower bones entering 

 into the formation of the ear-chamber is a distinct bone. No. 4, (Fig. 

 10), and, for reasons already given, is regarded by the author as homo- 

 logous with the external occipital of the fish and is therefore similarly 

 numbered and named. In the lizard, python and alligator the 

 functions of the external occipital bone of the turtle, No. 4, (Fig. 10), 

 are, however, filled by what appears, in the adult skull at least, to be 

 a part of the ex-occipital bone, No. 2*, (Fig. 9). At an early period 

 of development in the snake and possibly in all reptilia, this part of 

 the ex-occipital exists as a distinct bone, notwithstanding that in later 

 life it may have coalesced to such an extent with the ex-occii)ital 

 that its original distinctness is entirely lost. If such be the case, 

 which is not at all improbable, then that part of the ex-occipital in 

 the alligator, No. 2*, (Fig. 9), entering into the formation of the in- 

 ternal ear-chamber, should lie regarded as the homologue of the ex- 

 ternal occipital No. 4, (Fig. 10), in the turtle. The only essential 



33 Windischmann. De Penitiori auris in Ampliibiis structura, 1831. 

 Owen, Phil. Trans., 1850. 



Stannius, Haudbuch der Zootomie, Zweite Aufl., Zweites Buch, 18.56, pp. 

 58, 164. 



