1910.] Elements of the Mammalian Skulls. 425 



border of orbit. Spreading forward over nasals and nmxillaries, encroach- 

 ing posteriorly on orbitosphenoids and alisphenoids, becoming expanded 

 transversely by the forward growth of the cerebrum. (The ])ostorbial con- 

 striction (see p. 419) is also gradually widened by the growth of the frontal 

 lobes.) Originally without postorbital processes, which appear to be 

 related in the Metatheria to the postorbital bar of the Cynodonts, but in (he 

 Eutheria to the necessity for a strong ridge adapted not only for the attach- 

 ment of the anterior portion of the temporal muscle but also to shield the 

 nerves and muscles of the orbit from the disturbing action of the temporal 

 muscle. 



Inicrpnrietab. The paired interparietals of Solenodon (p. 242) and 

 the unpaired interparietal of Marsupials, Rodents, Orycteropus, Galeo- 

 ■piihecus, Ungulates, etc. (p. 242) may be homologous with the large "parie- 

 tale mediale" described by Van Bemmelen in Ornithorhynchus; while the 

 parietals of normal mammals may possibly be homologous with the "parie- 

 talia lateralia" of the same form. The interparietals may share in the 

 formation of the tentorial plate (Weber). A large interparietal appears in 

 Oudenodon (Broom). p^ 



Siipraoccipital. The supraoccipital, which is a cartilage bone is usually 

 unpaired, but it is paired in Echidna (Van Bemmelen), and it arises from 

 paired centers in Tatnsia, Erinaceus and the Cetacea (Weber, 1904, p. 49). 



Orhitosplienoids. Preformed in cartilage. Very large in Prototheria, 

 reduced in many Theria. Probably not pierced originally by optic foramen; 

 overlapped on the internal surface of the brain case by the backward ex- 

 tension of the ethmoid. Marsupial and Placental types compared above 

 (p. 245). 



Alisphenoids. Preformed in cartilage. Large in Monotremes and 

 Marsupials, reduced in Placentals. In ^Marsupials entering the glenoid, 

 forming ])art of the bulla and extending well up on the side of the brain case. 



Basioccipital. Exceedingly short in Cynodonts. Short in IMarsupials 

 and Insectivores. Progressively lengthening in Creodonts and Carnivores. 



Basis phenoid. The pituitary fossa in Marsupials and certain Insecti- 

 vores has the posterior clinoid process reduced or wanting (p. 245) . 



Squam,osal. Primitively separated from the brain case by a post- temporal 

 canal (Monotremes, Theriodonts). Glenoid fossa perhaps originally located 

 about as in Marmosa, assuming a relatively posterior position by the forward 

 extension of the mid-cranial region in Didelphis and Thylacynus (see above, 

 p. 218); assuming a relatively anterior position by the backward extension 

 of the base of the cranium in Placental Carnivores, Primates and primitive 

 Ungulates. 



Postglenoid process. In Cynodonts, in certain jSIesozoic oNIarsupials 



