THE SKULLS OF MAMMALIA. 251 



Those which are presented bv what niav be termed aberrant 

 Mammalian skulls, e.g., the crania of the Monotremata and 

 Proboscidia, and of the aquatic Mammalia — the Sirenia, Phocidast 

 and Cetacea. 



I am not aware that there is any example among the Mam- 

 malia of the bones of the roof, or lateral walls, of the two pos- 

 terior segments of the skull taking a share in the formation of the 

 floor of the cranial cavity. On the other hand, a careful study 

 of development will probably show that it is no uncommon 

 circumstance for the orbito -sphenoids to unite together in the 

 middle line, so as to exclude the presphenoid from the cranial 

 floor, or even to supply its place entirely. 



A still more remarkable deviation from the typical arrange- 

 ment than this occurs in certain Mammals, and has been thus 

 noted by Cuvier (Lecons ii., p. 319) : — "La lame cribleuse de 

 l'ethmoide dans tous les Maids, dans les Loris, et les Galagos, 

 vient toucher comme dans l'homme, au sphenoide anterieur; 

 tandisque, dans les Singes, elle en reste eloignee en arriere par le 

 rapprochement des deux cotes du frontal." 



I find the union of the frontals to which Cuvier refers in this 

 passage to take place in Cynocephalus, Macacus, Cercopithecus, 

 and Semnojnthecus. The frontals, however, do not really separate 

 the presphenoid and ethmoid, but only form, above the junction 

 of these two bones, the front part of a thick osseous bridge, the 

 hinder part of which is contributed by the orbito-sphenoids. 



The Gorilla agrees with the Monkeys and Baboons in these 

 respects. Thus, in the adult male Gorilla in the Museum of 

 the Koyal College of Surgeons, the distance from the anterior 

 boundary of the sella turcica to the anterior end of the cribriform 

 plate of the ethmoid is 14 in. Of this extent of the base of the 

 skull, 0*35 in. is occupied by the conjoined orbito-sphenoids, 

 042 in. by the coalesced frontals, and 0*6 in. by the lamina 

 peiyendicularis of the ethmoid. But, in a vertical section, the 

 ethmoid is seen to extend back under the basi-cranial processes 

 of the frontals (which are not more than one-fifth of an inch 

 thick) as far as the suture between the orbito-sphenoids and 

 these processes, which end anteriorly in a free rounded, trans- 



