No. I.] THE VERTEBRATE EAR. j^ 



einem horizontalen, einen frontalen oder sagittalen Bogengang 

 sprechen. Auch beim Menschen kommen solche abweich- 

 ungen von den verschiedenen Ebenen, und zwar konstant, vor," 

 u.s.w. ; and he cites the passage in Henle's SplancJinologie, 

 deahng with measurements of the angles of divergence. Henle 

 gives io° divergence from the assumed normal planes as the 

 most constant (normal) condition in the human subject. 



In the Sting Ray the posterior canal is almost entirely sep- 

 arated from the sacculus, and has no relation to either of the 

 remaining canals. Its means of communication with the saccu- 

 lus is a very small tube, given off from the canal above the 

 ampulla, as already described (Fig. i). The same condition 

 of separation has been figured by both Hasse and Retzius for 



Cut 4. — A projection of the semicircular canals of the ear of Dasyatis centrurus 

 on the horizontal plane, to show their angular relations with the vertical (sagittal) 

 plane. 



other Elasmobranch forms, and is one of the characters of 

 special importance, which we shall need to consider again. 



The anterior and external canals, which unite into a common 

 tube (PI. I, Fig. i) above, are only connected with the utriculus 

 by a very small hollow stem. 



It is thus apparent that the channels, placing the utricular 

 and saccular canals in communication with their respective 

 chambers, are very much simpler than those present in Tor- 

 pedo (Cut i). 



The separation of the posterior canal progresses in some 

 instances to a complete separation from the sacculus, the com- 

 municating tube becoming closed and persisting as a solid 



