2i8 THE SKULL. [chap. 



position of the cribriform plate, are mostly placed verti- 

 cally. 



The pterygoids {Pt) are flattened, horizontal, oval plates, 

 attached to the obliquely truncated postero-external extremi- 

 ties of the palatines, and form part of the floor and the inner 

 wall of the tympanic cavity, an arrangement not met with in 

 any other Mammal. The tympanic (7)') is a very slender 

 ring, incomplete for a small space at the upper and outer 

 end, and does not become ankylosed to the periotic. The 

 latter (^Per) is large, has no fossa for the flocculus, sends out 

 a large expansion (pterotic), forming a portion of the cranial 

 wall between the squamosal, parietal, and occipital, the 

 lower and hinder part at least of which corresponds with 

 the mastoid portion. 



Each ramus of the mandible is a mere slender style, with- 

 out any ascending portion, and with but rudiments of 

 coronoid process and angle. The condyle is very small, 

 elongated from before backwards, and very narrow. 



Fig. 71. — Lower surface of liyoid of Echidna {Echidna hystn'x). eh epihyal 

 ch ceratohyal ; l^h basihyal ; t/i thyrohyal. 



The hyoid (Fig. 71) has a well-ossified, transversely 

 extended, flattened and arched (with the concavity for- 

 wards), basihyal {b/i). The anterior cornu has only two 

 ossifications {ch and ch), apparently the epi- and cerato-hyal, 

 as the upper one has a long ligamentous connection with 



