860 Tsnt. F. G. PARSONS ON THE [Nov. 29, 



The temporal fossa in both auimals is ridiculously small, and is 

 separated from the orbit by a postorbital process, which is much 

 better marked in Dipus than in Pedetes and is altogether absent in 

 the foetus. The squamosal is remarkable for a backwardly pro- 

 jecting process which locks it into the periotic bone ; this spur is 

 simple in Pedetes, but in Dipus it is H-s]:aped, a vertical bar 

 extending at right angles from the hinder end of the primary hori- 

 zontal one. The periotic has the usual tympanic canal running 

 upward and backward from the laterally compressed tympanic 

 bulla. In Dipus the canal is extremely short, and in the foetal 

 Pedetes there is merely a tympanic ring. Above the external 

 auditory meatus in both animals, the supratemporal bulla gives the 

 characteristic swollen appearance to the hind part of the skull. In 

 the foetus no bull as are present ; the jieriotic is a mass of cartilage 

 in which the pro-, epi-, and opisthotic ossifications can be seen. 

 The backward projection of the squamosal is, however, quite ossified. 



On the ventral surface of the skull the incisors are perfectly 

 white and quite smooth in Pedetes ; in Dijms they are also white, 

 .but there is a single longitudinal groove in them. In the foetal 

 Pedetes it is interesting to notice that the incisors, which are just 

 appearing, are also grooved. 



The anterior palatine canals are slit-like and not very large in 

 either animal ; in Pedetes they are situated at the bottom of a rather 

 deep fossa. The bony palate is one of the chief points of difference 

 between the two skulls : in Pedetes it reaches as far back as the 

 first molar tooth, in Dipus it extends considerably farther back 

 than the last molar. In the foetus there are rudiments of three 

 teeth on each side, presumably the premolar and first two molars ; 

 the most anterior of the three is the one best developed. 



On a level with the hinder edge of the internal pterygoid plate 

 there is in Pedetes a small median opening in the basioccipital bone ; 

 this communicates with the foramina rotunda, but does not open 

 directly into the cranial cavity. More posteriorly in the mid- 

 line of the basioccipital bone is a round aperture, large enough to 

 admit a wax vesta match ; in the recent state this was closed by 

 membrane. In the foetal specimen both these openings are present, 

 but they are bilateral instead of median. In Dipus neither is present. 



In the mandible the chief difference between the two animals 

 is that in Pedetes the symphysis, although not synostosed, is 

 immovable ; while in Dipus the incisors are capable of separation 

 and approximation as in most mj^omorphiue rodents. In addition 

 to this the angular process is much larger in Dipus than in Pedetes 

 and is perforated by an oval foramen. The lower incisors too of 

 Dipus are much more laterally compressed than they are in 

 Pedetes. 



The Atlas of Pedetes is remarkable for having on each side three 

 foramina for the vertebral artery ; there are the usual two in the 

 transverse process and dorsal arch, and an additional one formed 

 by a small bridge of bone arching over the groove for the artery mid- 

 way between the other two. In Dipus the one in the transvers 



