1 1 2 ELEMENTAR Y ANA TO MY. [LESS. 



however, it remains, for the most part, pretty much as it is 

 in him. 



The os planum, in that it is large and conspicuous and 

 situated on the inner wall of the orbit, presents conditions 

 which man shares with the Apes, but which are very rare in 

 other forms. In some Cats, however, it appears in the orbital 

 between the lachrymal, frontal, and palatine bones. 



The part corresponding to the os planum of man exists 

 within, and hidden by, the maxillary plate in other mammals ; 

 but it may be represented by a cartilage or cartilages, as in 

 birds. 



The lateral masses of the ethmoid appear in the form 

 of distinct bones (pre-frontals] in Crocodiles, Lizards, and 

 osseous Fishes. They may, as in the Chameleon, extend on 

 to the maxilla, so shutting out the nasals from the anterior 

 nares ; and in some Chameleons they may be prolonged so 

 as to help to form two great bony horns projecting forwards 

 from the muzzle. 



The median ethmoid is constantly present in a cartilaginous 

 or osseous condition as part of the internasal septum. Even 

 in fishes it is generally more or less ossified. It is possible 

 that it may appear on the external surface of the skull in 

 Mammals, as in the Seal Monachus. It does so in Fishes 

 (e.g. Carp and Tench), and at least in some Birds. 



A peculiar condition of the parts may exist, such as is found 

 in the Frog, where the ethmoid forms (or is part of) a bone 

 which has been likened to a dice-box with a vertical parti- 

 tion at one end, and has been named by Cuvier os en ceinture, 

 or girdle-bone. It consists probably of the lesser wings of 

 the sphenoid united to the median and parts of the lateral 

 ethmoid. 



The ethmoidal cells of the lateral ethmoid may attain a 

 much greater size and complexity than in man, as we see in 

 the Dog and very many Mammals. On the other hand, these 

 parts may utterly and entirely abort, as in the Porpoises. 



A cribriform plate is common to, and generally large in 

 man's class ; it may be wanting, as in the Porpoise, or enor- 

 mous, as in the Echidna. 



The crista galli is more denned in man than generally 

 in Mammals, but it may be very large, as in some Seals and 

 Ungulates. 



In certain Seals again, as in Cystopkora, and in the Tapir, 

 the median ethmoid may extend forwards beyond the anterior 

 end of the nasals. 



