xix. 9 SKULL OF MONOTREMES 551 



present, unlike those of any other mammals (Fig. 325). These true 

 teeth are replaced by horny structures, formed by an ingrowth of 

 epidermis beneath them and apparently used for breaking the shells 

 of the molluscs. It is particularly unfortunate, since most of our 

 knowledge of early mammalian affinities comes from their teeth, 

 that we can deduce so little from those of the monotremes. 



bas. cpisth. 



Fig. 326. Skull of platypus seen from below. 



dl. alisphenoid; has. basioccipital; ep. epipterygoid; ex. exoccipital; glen, glenoid facet of 

 squamosal; jug. jugal; max. maxilla; opisth. opisthotic; pal. palatine; pm. premaxilla; 

 pv. prevomer; sq. squamosal; v. 'vomer'- — basisphenoid. (From Ihle, after van Benneden.) 



The skull (Fig. 326) is specialized in both genera, particularly at the 

 front end, and many of the bones fuse early. There is a wide com- 

 munication between the orbit and temporal fossa. There are many 

 'reptilian' features; for instance, separate pterygoid bones. The 'dumb- 

 bell-shaped bones' are perhaps the remains of the prevomer but may 

 be vestiges of the palatine processes of the premaxillae. Small pre- 

 frontal and postfrontal bones are present. In the temporal region there 

 is a narrow canal that apparently represents the posterior temporal 

 fossa of Therapsida. A curious feature is that the lateral wall of the 

 brain-case is formed by an anterior extension of the petrosal and 

 not by the alisphenoid. 



