SKELETON OF MONOTREMATA. 319 



by the exoccipitals,fig. 203, 2,2, which are separated above the fora- 

 men magnum by a notch, ck^sed by membrane, in the recent state. 

 The superoccipital, ib. 3, is a transversely oblong quadrilateral 

 plate, articulated by ' harmonia,' not only with the exoccipitals, 

 but Avith the large mastoids, 8, and anteriorly with the parietals, 

 fig. 197, 7. The basisphenoid, fig. 202, 5, supports laterally a 

 pair of alisphenoids, fig. 197, 6, w^hich are notched posteriorly by 

 the trigeminal nerves, and expand as they rise to articulate mth 

 the parietals, 7, the mastoid, 8, and anteriorly with the orbito- 

 sphenoid and frontal, 1 1 . The mastoid, 8, is chiefly conspicuous by 

 its great size, in the Echidna, and the share Avhich it takes, con- 

 jointly with the petrosal, in the formation of the lateral, lower and 

 posterior parts of the cranial cavity : in this character it retains 

 much of its ornithic condition, fig. 196, s. The small vacuity, 

 left in the Monotreme, between the mastoid and alisphenoid, is 

 closed by the application thereto of the posteriorly expanded squa- 

 mosal, fig. 197, 27. The pre- 203 

 sphenoid, fig. 202, 9, is con- 

 nate with orbitosphenoids, fig. 

 197, 10, pierced by the small 

 optic nerves : the frontals, 1 1 , 

 expand as they rise, but with- 

 out developing superorbital 

 ridges, and meet at a toothless 

 suture along the middle of the 

 narrow forehead. The vomer 



, f, , 1 • n Occiput of Echitlua. lxxix'. 



and preirontals are chieny 



remarkable for their connection with enormous and obscurincr tur- 

 binals, supporting an olfactory organ of vast extent. The anterior 

 part of the frontals is largely overlapped by the bases of the nasal 

 bones, which encroach upon the interorbital space. These, fig. 

 197, 15, receive the upper edge of the maxillary into a groove at 

 their outer margin, and articulate anteriorly with the premaxil- 

 laries, ib. 22, which meet above the nasal canal in front of the nasal 

 bones for an extent of about three lines, and thus exclusively 

 form the boundary of the single, oval, and terminal external 

 nostril. The lower or palatal process of the premaxillary 

 extends backward in the form of a long and slender pointed 

 process which is wedged into a fissure of the maxillary. The 

 incisive fissure is narrow and extends from the premaxillary 

 symphysis some way between the palatine plates of the maxil- 

 laries. The palatines, fig. 202, 20, arc long and entire Avliere 

 they form the hinder half of the roof of the mouth, diverging 



