. The elasmobranch fishes . position posterior to the apertures for the endolymphatic duets. A sagittal section through the cranium of the skate as given by Howes (1883) shows that each fenestra enters a well defined cavity or tympanum (ty., fig. 235b) within the ear capsule, and further (fig. 235a) that the aperture is closed dorsally by a membrane (nih.). Over the membrane is a semifluid layer (fl.), above which is the integument. In a type in which the head is flattened (ray) this mem- brane fits closely over the fenestrae, and may serve to transmit sound waves to the cavity below it and th
. The elasmobranch fishes . position posterior to the apertures for the endolymphatic duets. A sagittal section through the cranium of the skate as given by Howes (1883) shows that each fenestra enters a well defined cavity or tympanum (ty., fig. 235b) within the ear capsule, and further (fig. 235a) that the aperture is closed dorsally by a membrane (nih.). Over the membrane is a semifluid layer (fl.), above which is the integument. In a type in which the head is flattened (ray) this mem- brane fits closely over the fenestrae, and may serve to transmit sound waves to the cavity below it and thus to act some- thing after the fashion of the tympanic membrane of the middle ear of higher forms. The endolymphatic duct (, figs. 236 and 237) is normally small proximally and then enlarges into a more or less tor- tuous endolymphatic sac (). In Squa- iina, on the contrary, and to a certain extent in Acanthias, the mouth of the duct is enlarged and although bent at its upper part it is little changed in caliber throughout its course. At its base the duct broadens out into the sacculus (s.). In some forms this is relatively small (Alo- pias: Heterodontus, fig. 236c), but it may be of large size (Sqnalus sucMii, fig. 236a, s.). It is within this cavity as well as in the utriculus that the otoliths or so-called ear stones are lodged. These in Sqnalus con- sist of a mass of calcareous material con- tained within the endolymph. In some of the other forms they are small, and in the embryo of Squatina they are apparently absent. In the adult Squatina a most interesting condition is reported. Here it is said that the place of the concretions of other forms is taken by sand grains which enter the wide endo- lymphatic duct. From the inferior and posterior angle of the sacculus (figs. 236 and 237) the lagena (l.) arises. This is usually a tongue-like projection as in Heptanchus, but it may assume a form greatly enlarged at the end, as in Lamna. The lagena is that part of the ear
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