354 



MECHANICAL AND ACOUSTICAL SENSES 



point is a depression, the parietal fossa, which is filled with connective tissue 

 and on the floor of which is an opening, the fenestra ovalis, closed on the 

 side of the fossa by a membrane and, within the skull, by the wall of the 

 posterior vertical canal. This specialised arrangement suggested to Tester etal. 

 (1972) a role in sound detection, in which the endolymphatic duct would 

 function in pressure equalisation. 



Numerous accounts of the form of the easily dissected elasmobranch laby- 

 rinth are available in the literature, beginning with the excellent descriptions 

 by Retzius (1881). Stewart (1905, 1906) characterized eight species not 

 illustrated by Retzius, and Werner (1930) reviews all the literature and 

 figures 21 species selected from the main elasmobranch families. Additional 

 details are found for Squalus in Vilstrup's monographs (Vilstrup 1950, 1951) 

 and for Carcharhinus in Tester et al. (1972). Good descriptions of develop- 

 ment are provided for Mustelus by Ayers (1892) and, especially, by Quiring 

 (1930) for Squalus. 



From his review Werner (1930) appreciated that the elasmobranch laby- 

 rinth is found in two basic forms. In one, as in Carcharias, there is a small 

 sacculus, a long endolymphatic duct, and a crus commune formed from 

 adjoining vertical canals (Figure 12B); in the other, there is a large sacculus, 

 and although the anterior vertical and horizontal canals share the same por- 

 tion of the utriculus, the posterior vertical canal is separate. This form is 

 much more common and is illustrated by Lamna (Figure 12A). 



B 



Figure 12 Two examples of elasmobranch labyrinths seen from the medial view, with the 

 anterior vertical canal on the left (A) Lamna: (B) Carcharias (redrawn from Stewart 1905). 



