l82 VERTEBRATE SKELETON 



hardly noticeable in lower mammals, is marked in the higher where 

 the foramen is on the ventral side of the skull. Growth of the cere- 

 brum forwards causes it to override the olfactory nerves so that the 

 olfactory foramina are vertical in the higher groups, the anterior 

 part of the cranial cavity lying dorsal to the olfactory labyrinth. 

 The dorsal extension of the brain causes a vaulting of the roof, 

 paralleled only in birds. 



The number of bones is smaller than in most lower classes, the 

 reduction being partly due to fusion, partly to absolute loss. Thus 

 the otica are fused to a petrosal bone and this is often united with 

 squamosal and tympanic, forming a temporal bone. The sphenoi- 

 daha unite to a single sphenoid bone (sometimes two), and only a 



Fig. 194. — Cranium of Tupaia (Wortmann, '20) showing bones possibly to be 

 interpreted as he has. /, interparietal; /, lacrimal; p. parietal; po, postorbital 

 prf, prefrontal; qj, qtiadratojugal; 5, squamosal; s, zygomatic. 



single bone, largely dentale, is recognizable in either half of the adult 

 lower jaw. Of actual cases of loss, the supratemporal, pre- and post- 

 frontal, postorbital and quadratojugal may be cited, no traces of 

 these having been seen.^ 



Most important of the differences between the skulls of mammals 

 and those of lower Vertebrates is the disappearance of the quadrate 

 from the suspensorial apparatus; a position it occupies from Elasmo- 

 branchs to birds; in mammals it functions as an element of the 

 sound-transmitting structures of the middle ear, the lower jaw in 

 mammals being articulated with the squamosal, and the quadrate 

 is removed from the hinge. 



' The recent statement of Wortman ('20) that pre- and postfrontal and quadrato- 

 jugal bones (fig. 194) occur in adult mammals needs confirmation. Monotremes 

 have an element in the position of the postfrontal which maj- be this bone. It is pos- 

 sible that the mammalian lafrirpal is the same as the reptilian prefrontal, but this has 

 been noted (p. 139) as being at least doubtful. In a few cases a bone has been 

 described (.\meghino, Fuchs) in the zygomatic arch in the position of a quadratojugal, 

 but it has not been shown to be such. 



