1910.] MtiNitiialitiN Skull: Fonimitia. 429 



llic Cranial Furamina. 



Most of the cranial foramina are pretty constant in position throughout 

 the Theria (except in the Cetacea). But certain foramina that are variable 

 either in presence or position afford distinctive ordinal characters, so that 

 many orders and often certain families, may be distinguished by one or 

 more foraminal characters {cf. Monotremata, p. 150; Marsupialia p. 222; 

 Insectivora p[). 253, 285, Rodentia, p. 329, Creodonta p. 306, Condylarthra 

 p. 354, Proboscidea p. 367, " Protypotheres, " Primates p. 321 , etc.). For the 

 convenience of the student a brief review of the cranial foramina is given 

 below: ^ 



I. Foramina for the cranial nerves. 



Fenestroe cribrom'. Nerve I. Lacunae left in the cribriform plate; 

 rarely {Ornithorhynchus , Tarsius) confluent intt) one or two openings. 



Optic foramen. Nerve II. Apparently this nerve originally issued 

 through the sphenorbital fissure (foramen lacerum anterius) {cf. Mono- 

 tremata, Marsupialia, Sorer). It later ])ierced the orbitosphenoid (most 

 Placentals). 



Sphenorbital foramen (foramen lacerum anterius). Nerves III, IV, Vj, 

 VI, and sometimes II. Constant. Opposite foramina lacera anterius 

 confluent in Marsupials. 



Foramen rotundum: Nerve Vj. Ner^■e originally issued through 

 sphenorbital fissure (Monotremes). In Marsupials generally a prominent 

 opening some distance behind the foramen lacerum anterius. In Placentals 

 the alisphenoidal partition separating it from the foramen lacerum anterius 

 is often poorly developed {e. g., Arctoniij.s-), this causing more or less con- 

 fluence Avith the foramen lacerum anterius. In the Placentals the flange of 

 the alisphenoid which is external to the foramen rotundum often grows 

 forward and forms a common arcade into which the rotundum and foramen 

 lacerum anterius open. 



Foramen ovale. Nerve V3. Originally issuing through the foramen 

 lacerum medius but enclosed by the progressively increasing alisphenoid 

 (Weber). The foramen ovale always looks toward the inner face of the 

 mandible, since it gives exit to the mandibular branch of the trigeminus 

 nerve. In IMarsupials it is generally in or near that portion of the alisphenoid 

 which extends on to the glenoid fossa. In Placentals it is sometimes second- 

 arily confluent with the foramen lacerum medium {e. g., Rhinoceros, Equus, 

 many Rodents, Cetacea). 



1 A very clear treatment of the foramina in the Dog is given in Reynold's ' The Verte- 

 brate Skeleton,' pp. 399-402. Weber (pp. 47-52) reviews the mammalian foramina in general. 



