1926] Hall: Skull of the Bodent OtospermophiJus grammnrus hcechei/i 379 



mota, where three condylar foramina appear on the external face of 

 each exoccipital. The exoccipitals fuse with the basi- and supra- 

 occipital between M and Q. 



Supraoccipital. — The large fissura bisupraoccipitalis suggests that 

 this bone may have come from two centers of ossification. Whether 

 or not this is the case figure 24 shows that in A ossification is certainly 

 more extensive in the two lateral portions than in the central region. 

 The interparietal is closely associated with the 

 supraoccipital as early as ^. In F ossified 

 portions of the i)arietals, squamosals and 

 mastoids come into contact with the supra- 



Fig. 24. Posterior view occipital. In D it leans slightly forward from 



or supraoccipital in J ^ & j 



showing fissura bisupra- the vertical axis, but between E and F it 



occipitalis extending dors- . , . . -r ^ t 



ally from foramen mag- assumes a more vertical position. In G and 

 num. X 7. jj^ jj j^ again leans backward from a line 



drawn perpendicularly to the plane of the palate. From H on, its 

 forward inclination increases. The change that occurs after H is 

 very probably the direct result of muscular strains which, in fos- 

 sorial and aquatic forms, very often tend to decrease the basicranial 

 angle, that is, the occiput becomes inclined forward. Spalax and 

 Balaenoptera represent extreme cases of development in this respect. 

 Two lateral swellings mark the positions of the lateral lobes of 

 the cerebellum. However, in some skulls of Q the external face 

 becomes extremely rugose and the bone itself much thickened, pre- 

 sumably from the etifect of muscular pull rather than from brain 

 pressure ; thus muscles as well as the brain appear to have consider- 

 able to do Avith the final form of the supraoccipital as well as with its 

 inclination with respect to the plane of the palate. Externally a 

 median vertical keel is present from J onward. An increase in height 

 from 2.5 mm. in C to 10.0 mm. in Q, or 300 per cent, is made. 



Interparietal . — In A and B a striking feature of the interparietal 

 is the relatively large area it covers in the roof of the skull. This is in 

 marked contrast to the conditions shown by Parker (1885) of Centetes 

 (pi. 32, fig. 1) of Rhynchocyon (pi. 36, fig. 1) and of Bos by AVilhelm 

 (1924, text figs. 1-7). In these eases the parietals appear to contribute 

 a relatively greater share of the skull's roof in the embryonic stages 

 than they do in Otospermophilus. In A and B ossification has hardly 

 begun. In C the interparietal is well ossified. While no distinct suture 

 is present that separates the bone into two parts, as is the case in the 

 skull of a young Erethizon that lies before me, there are indications 



