204 



VERTEBRATE SKELETON 



quadrate has slowly disappeared from the hinge, leaving the lower jaw suspended 

 by the squamosal, a view negatived by the fact that the articulare is always 

 formed from the hinder end of the Meckelian cartilage, while in mammals the 

 Aleckelian never approaches the glenoid fossa. It has yet to be proved that 

 the mammals have descended from streptostylic vertebrates, though the close 

 association of squamosal and incus in some species looks that way. Another 

 theory is that the quadrate is the homologue of the tympanic bone, a view which 

 ignores the fact that quadrate is cartilage, the tympanic is dermal in origin. 



All theories so far advanced have to meet a serious difficulty for which no 

 explanation has been advanced. That is how the hinge has shifted from the 



posterior end of the Meckelian 

 (articulare) and quadrate to the con- 

 dylar squamosal articulation of mam- 

 mals, a shift which must involve a 

 time when there were tw6 hinges to 

 the jaw, both functional at once. 



Scarcely anything is known 

 of the skulls of the lowest and 

 oldest orders of mammals, 

 except the teeth and the asso- 

 ciated parts of the jaws. There 

 must have been mammals ear- 

 lier than the upper triassic 

 forms known, of which no traces 

 have been discovered. 



MONOTREMATA have skulls prim- 

 itive in some respects, specialized in 

 others. The perforation of the 

 basisphenoid by the internal carotid 

 artery, the identity of optic foramen 

 (also of foramen rotundum in Ech- 

 idna), and orbital fissure, the incom- 

 plete tympanic ring and no true 

 tympanic cavity, a separate septo- 

 maxillary in the embryo and the 

 simple olfactory foramen of Oniii/iorhynchus are primitive or embryonic 

 features; while the absence of most normal sutures, the absence of teeth and the 

 horny sheath of the jaws belong in the second category. 



Among other features are the common foramen for nerves IX-XII, the small 

 zygomatic (a small protuberance on the frontal in Ornithorhynchns) and its 

 absence in Echidna, the zygomatic arch being formed by maxilla and squamosal; 

 and the prevomer in Ornithorhynchus . The nares are nearly terminal in Echidna, 

 farther back in the duckbill. The occipital condyles of Echidna are connected 



Fig. 214. — Cranium of Echidna (Van 

 Bemtneln, '01). a, alisphenoid; bo, basiocci- 

 pital; e, ethmoid; bs, basisphenoid;/, frontal; 

 tn, maxilla; ms, mastoid; p, parietal; pf, 

 postfrontal; pi, palatine; pm, premaxilla; pt, 

 pterygoid; so, supraoccipital; sq, squamosal. 



