Special Osteology. Rodentia. 35 



Kygomatic arch. Anteriorly to each orbit there is usually present a 

 wide vacuity, termed the antorhital for amen y of variable size ; in Ilela- 

 mys Capensis (Cape Jerboa) it is larger than the orbit. An interorlital 

 foramen is also frequently found; the common outlet of the optic 

 nerves {e. g. Lepus) extending forward so as to occasion a small 

 vacuity at the back part of the interorbital septum. The temporal 

 and orhital fossce are Mended together in all Ilod-entia. The naml 

 ^ndi frontal bones are frequently large and long, both being especially 

 developed in Hystrix Cristata (Porcupine). Osseous hullm are gene- 

 rally present, formed by the petro-tympanic. In most Eodentia the 

 anterior part of the vomer articulates with the median ascending 

 process of the praemaxillary, arching over the wide vacuities which 

 lead from the nasal passages to i}iQ prepalatine aperture. The lower 

 jaw is modified for the lodgement of the pair of long scalpriform in- 

 cisors, the sockets of which may extend to the middle (Lepus) or 

 even to the hind part (Castor, Hystrix) of the ramus. In all Hodents 

 the condyle is convex transversely and extended longitudinally 

 (? Lepus) in which direction it is chiefly moveable, to give a full 

 action to the gnawing incisors : there is sometimes found (^e.g. in 

 Capybara and Cavia Cobaya) a process directed backward from the 

 angle of the jaw. 



The Vertebral Column. The neural spine is usually longest in the 

 second and seventh cervical vertebrse : it is obsolete in the interme- 

 diate cervicals in many Rodents. In all the agile, flexible bodied 

 Rodents the neural spines incline towards the 11th (? Lepus) dorsal, 

 the spine of which is vertical, and indicates the centre of motion. 

 The thoracic rils consist of bony pleur-,and gristly ha^m-apophyses. 

 In some Rodents, the Ensiform process is expanded behind into 

 a thin cartilaginous disc. Throughout the lumbar region both an- 

 and met-apophyses are generally well developed ; long hypapophyses 

 are also developed. In most Rodents with long tails h^emapophyses 

 are developed beneath the intervertebral spaces, e.g. Dipus (Jerboa). 



Limh ho?ies. There are few generalizations deducible from the 

 limb bones ef Redentia, since the habits of the several species com- 

 posing this order are so various ; some have limb-s giving power in 

 running, some in swimming, some in burrowing, some in leaping, 

 some in climbing, and a few show modifications in relation to para- 

 chute-like expansions of integument for a kind of flight. The Clavi- 

 cles are frequently represented by slender ossicles freely suspended {e.g. 

 in Lepus timidus, Dasyprocta, Capybara, and Hystrix): when this 



