1910.] The Rodent Skull. 329 



with a normal tritubercular dentition; but nevertheless it may indicate a 

 very ancient separation from the Insectivores ; and even although Dr. Woi-t- 

 man's evidence be wholly insufficient, his conclusion that the trituberculy 

 of Rodents is partly secondary must be admitted as a possibility not yet 

 dis})roven. 



General morphologi/ of the Rodent sicull. — The evolution of the scalpri- 

 forni incisors and of the jaw action described above (p. 328) has doubtless 

 conditioned: first, the development of the elongate mandibular condyles 

 and glenoid sockets, and secondly, the readjustment and modification of 

 nearly all the bones of the skull; so that among known Rodents relatively 

 few points of resemblance to the typical Placentals are left. 



The cranial foramina (examined by the writer in Lepus, OcJiotona, 

 Sciurus, Arctomys, Mus, Hystrix, Cavia) conform in general to the normal 

 Placental type. The orbito-sphenoid is pierced by a large optic foramen; 

 the lachrymal, infraorbital, sphenopalatine, ethmoid, anterior and posterior 

 palatine foramina, the foramen lacerum anterius, foramen rotundum, 

 foramen ovale, eustachian opening, foramen lacerum posterius, condylar, 

 post-glenoid, stylomastoid, and mastoid foramina are all present. The 

 alisphenoid canal (occasionally absent) is apparently not the large tunnel 

 leading forward to the foramen lacerum anterius but a small canal external 

 to it. The foramen ovale is also prominent, except in Lagomorphs and 

 Hystricomorphs, where it appears to be confluent with the foramen lacerum 

 medius. The basisphenoid is perforated by a transverse canal, which, as in 

 Didelphis and certain Insectivores (p. 291) is stated by Tullberg (Weber, 

 1904, p. 474) to transmit a vein. The entocarotid and its stapedial branch 

 are variously developed (van Kampen, 1905, p. 566). When the former is 

 well developed it often runs through a canal or groove lying between the 

 bulla and the petrosal and enters the cranium through the foramen lacerum 

 posterius. In Lepus it pierces the bulla. The condylar foramen, as in 

 certain Insectivores, is double or even triple (some specimens of Arctomys), 

 perhaps in correlation with the primitively composite nature of the hypo- 

 glossus nerve (C. S. Mead). The presphenoid is sometimes bounded on 

 either side by longitudinal vacuities. The elongate anterior palatine fora- 

 mina are often confluent inferiorly. Tympanic region. The lateral margins 

 of the basisphenoid are often raised and more or less excavated on each side 

 for the very large inflated tympanic bulla ; this fact tends to indicate that in 

 the case of Microgale and other Insectivores the functional participation of 

 the ali- and basisphenoid in the bulla is no indication of relationship to the 

 Marsupials but a result of the pressure exerted by the expanding bulla upon 

 the borders of the adjacent bones. The infraorbital foramen is invaded in 

 the higher Rodents by an internal anterior slip of the masseter muscle and 



