232 CLASS xv. 



angle of the eye, behind the two horizontal eye-lids. The third eye- 

 lid (membrana nictitans) and the lower have often a greater extent 

 of motion than the upper. With the third eye-lid a special lachry- 

 mal gland (glandula Harderi] is in connexion, as in birds. The 

 proper lachrymal gland, which lies on the outside of the eye-ball, is 

 often very large, especially in chelonians. The form of the eye- 

 ball usually differs much from that of a regular sphere. In the 

 chelonians and in most of the lacertine animals, at the anterior part 

 of the cornea, there is a ring of bony plates which support the eye- 

 ball. The lens is more or less spherical, in front however mostly 

 somewhat flatter than on the posterior surface. In many of the 

 lacertine animals a fold covered with black pigment runs obliquely 

 from the bottom of the eye-ball through the vitreous humour to the 

 lens, and corresponds with the pecten in the eye of birds 1 . The 

 pupil of the eye is mostly round; in the crocodile it is an elongate 

 rhomboidal fissure. 



The organ of hearing presents a gradual scale of development 

 from the simple structure of fishes to that of birds. The labyrinth is, 

 as in the cartilaginous fishes, distinct from the cavity for the brain, 

 and is inclosed in the petrous bone, or often in a part of the occipital 

 bone also. The vestibule is filled with a whitish fluid, containing 

 microscopic crystals of carbonate of lime. There are three semi- 

 circular canals and afenestra ovalis (foramen vestibuli) present; in 

 the serpents, lizards and chelonians (the haplopnoa) there is in 

 addition the commencement of a cochlea and a fenestra rotunda 

 (foramen cochlece). The cochlea appears to be most developed in 

 the crocodiles and formed almost as in birds; a membranous parti- 

 tion, extended between two cartilages, divides the internal space of 

 the cochlea into two cavities (scala tympani and scala vestibuli). In 

 both these fundamental forms of the auditory organ, in the imperfect 

 form of the diplopnoa and in the higher of the haplopnoa, a tympanic 

 cavity may be present or may be absent; where there is a tympanic 

 cavity, there is at the same time a tube which conducts from it to 

 the pharynx (tuba Eustachii). This is the case in most of the frogs, 

 where the tube is very wide ; in some tailless Batrachii and in all 



1 This production is figured by D. W. SCBMMERRING in Monitor, De Oculorum 

 sectione horizontally Tab. in. In Iguana it is more developed, and forms two folds, 

 1. I. p. 60. 



