SKULL MAMMALS l8l 



MAMM^AXIA. — The skulls of mammals possess especial interest 

 since they include that of man, and consequently have been studied 

 more than those of any other class; yet there are many points await- 

 ing solution. The number and arrangement of the bones is fairly 

 constant, and yet there is as great a variety of form as in any non- 

 mammalian group. Then the question of the origin of the Mamma- 

 lia receives more information from the skull than from any other 

 structure. 



For many years the paired occipital condyles of mammals were 

 thought to indicate an Amphibian ancestry for the class, but it was 

 shown later that these could be derived from the tripartite condyle of 

 certain reptiles by the recession of the basioccipital portion. The 

 chondrocranium, while lacking an interorbital septum, can be 

 regarded as tropibasic, the loss of septum being the result of the great 

 size of the anterior part of the brain, but it must be recalled that 

 many Teleosts have tropibasic crania. Another feature of impor- 

 tance in this connexion is the comparatively entire oral roof, the hard 

 palate (palatine processes of premaxillas, maxillae and palatines) 

 being paralleled only in monimostylic reptiles. The bearing of the 

 quadrate on the problem of mammalian origin will be alluded to 

 later. 



There are other features of the skull which are the result of the 

 large brain of even the lowest members of the class. In other 

 Vertebrates, birds excepted, brain and spinal cord are in a straight 

 hne and the parts of the brain are small. Mammals have cerebrum 

 and cerebellum greatly enlarged relatively to other parts. As the 

 lower side of the brain is held in position by the exit of nerves and 

 the entrance of blood vessels through the skull, all expansions of the 

 brain must be dorsal to this, causing a widening and vaulting of the 

 lateral and roofing walls, the extent of which is usually directly 

 related to the size of the brain. (Some mammals — elephants, 

 Ruminants and Edentates — have cavities and cells in the diploe 

 which increase the size of the brain case.) The increase in size of 

 the cranial cavity afifects the ali- and orbitosphenoids, inconsiderable 

 elements in lower Vertebrates, and, as in birds, has brought the 

 squamosal into the cranial wall, and also has forced these bones and 

 the otica (vertical in lower groups) largely to the cranial floor. The 

 extension backwards of the brain has caused a change in the position 

 of the base of the skull and the foramen magnum, a change, while 



