in.] THE CRANIAL SKELETON. 137 



portion immediately above the cribriform plate forms a 

 more distinct chamber than exists in man. As we de- 

 scend through the mammalian class we often find it much 

 prolonged and of a large relative size, as in the Sheep, Pig, 

 and others, especially Marsupials, e.g. theThylacine. In some 

 forms, however, e.g. the Dolphins, the olfactory fossa is quite 

 wanting. Great difference in this respect may exist in nearly 

 allied forms e.g. in the Ornithorhynchus and Echidna, in the 

 first of which the cribriform plate is very small and bird-like, 

 while in the second it is enormously large. 



(2) The middle fossa is relatively larger in man than it 

 is in very many lower forms, but not in all, as we see by the 

 Dolphins. The foramina vary, as already noticed iii de- 

 scribing the several cranial bones. Very often the foramen 

 rotundum and sphenoidal fissure may be blended in a single 

 opening, as in the Squirrel ; or the optic foramen may form 

 but one aperture with the sphenoidal fissure, as in the Stag 

 and Opossum ; or the two optic foramina may be united into 

 a single median foramen, as in the Hare. 



The foramen ovale may be one with the foramen lacerum 

 anterius, as in the Horse. 



The optic foramina and sphenoidal fissures may together 

 be represented, on the inside of the skull, by a single opening, 

 as in the Swan ; and sometimes the optic foramen, sphenoidal 

 fissure, and foramen rotundum may be included in a common 

 aperture, as in the Booby. 



The sella turcica may be as sharply limited in other forms 

 as in man. On the other hand, the pituitary fossa may be 

 altogether unmarked, as in Batrachians. 



This fossa may dip down into a large canal running for- 

 wards and excavated in the basis cranii, as in many Fishes, 

 e.g. the Pike. This singular cavity has the cartilaginous floor 

 of the skull for its roof, while the para-sphenoid bone encloses 

 it below. 



(3) The posterior fossa may be much more sharply defined 

 than in man, as when a lamella of bone (consisting of an ossified 

 tentorium) is attached to the projecting border of the petrous 

 part of the temporal bone. The proportion borne by this 

 fossa to the middle one varies with the size and develop- 

 ment of the cerebellum which it shelters. Only in man 

 and a few Apes (notably the Squirrel Monkey, Chrysothrix) 

 can this fossa be said to form any considerable part of 

 the floor of the skull. The internal auditory foramen in 

 all man's class and In Birds is placed as in him, and there 



