602 CLASS XVII. 



occupied by a fibrous or cellular membrane, named tapetum, which 

 often presents lively colours with metallic reflections 1 . The lens 

 is commonly flat and is bounded posteriorly by a more convex 

 surface than in front. In the mammals that live in water, espe- 

 cially in the seals (Phocce), it is nearly spherical. As a rule there 

 are three eyelids present; but in man and the apes there is only a 

 vestige of the innermost perpendicular eyelid (the membrana nic- 

 titans), the so-named semilunar fold at the inner angle of the 

 eye 2 . 



The auditory organ of mammals differs from that of birds by 

 the labyrinth being more strictly included in the petrous portion of 

 the temporal bone, by a greater development of the cochlea (usually 

 forming two and a half or three spiral turns) , and by three auditory 

 ossicles, the malleus, the incus, and the stapes, which conduct the 

 vibrations of the tympanic membrane to the vestibule, on the 

 fenestra ovalis of which the plate of the stapes rests. This stapes 

 is in the Monotremes a long stile with a flattened base, like the 

 columella in birds; the transition to this is formed by the mar- 

 supiate and some edentate animals, where the two branches of the 

 stapes do not diverge, or diverge only when close to the terminal 

 plate, and in addition the ossicle terminates in a long stile. The 

 Monotremes also have only one ossicle in place of the malleus and 

 incus, and a very imperfect cochlea, which, as a conical appendage 

 of the vestibule somewhat swollen at the extremity, resembles that 

 of the birds and lacertians 3 . Generally an external ear also is 



1 Compare BRUEOKE in MUELLER'S Archiv, 1845, s. 387 405. 



2 Compare BLUMENTHAL Dissert, de exlernis oculor. integumentis, Berolini, 1812, 

 4to; TRAPP Symbolce ad anat. et pkysiol. Organorum bulbum adjuvantium, Turici, 1836, 

 4to (pp. 26 31, especially for the membrana nictitans and the included cartilage). 

 In most mammals with the membrana nictitans there occurs also a peculiar gland (Glan- 

 dula Harden, see above, pp. 232, 367), which we have already noticed in the reptiles 

 and birds. The lachrymal glands are often small ; in the cetaceans they seem to be 

 wanting, but in the dolphins a ring-shaped gland behind the eye-lids is present (K.APP 

 Cetaceen, s. 93). Besides the four straight and the two oblique muscles of the eye 

 there is in many mammals another running from the margin of the optic foramen to 

 the bulb of the eye, and forming that conical muscle which grasps it (musculus sus- 

 pensorius oculi); in the JBalcence (according to MATER Anat. Untersuchungen uber das 

 Auge der Cetaceen, Bonn, 1852, 8vo, s. 2) in the absence of the other muscles of the 

 eye it only is present. 



3 Tachyglossus, according to the investigations of HYRTL, is even destitute of 

 fenestra rotunda. 



