138 ME. W. K. PAEKER ON THE STRUCTTIRE AND 



structure of these parts is a sort of morphological halting between the ornithic and the 

 reptilian types ; but the spur of the basitemporal which stands out against the pterygoid 

 of the Lizard never articulates with that bone in birds, and in them is seldom developed 

 to any extent ; it is very large on each side, however, in the King Vulture, and in the 

 so-called Dmornis casuarinus, Owen, a gigantic pickaxe-headed Rail (see Zool. Trans, 

 vol. iii. pi. 52 for beautiful figures of this (1) extinct bird). On the other hand, the true 

 azygous basisphenoid is aborted in all the Lacertilia, and therefore those spurs of it 

 which culminate in the Struthionidse, exist persistently in nearly half the ornithic genera, 

 appear in the embryo of all, and reappear in the Cavies amongst the Mammalia, have 

 no existence in the Lizard-tribe. A misapprehension of the nature of these spurs, and 

 of their relations, has vitiated much that has been written upon this subject, and quite 

 prevented anatomists from educing a true harmony of the parts. 



The OS quadratum (Plate XL figs. 1, 2, q.) of this young Emu is wedged between and 

 packed amongst its surroundings much more like its counterpart in the Tortoise than 

 the same bone in the typical bird, so delicately hinged, and so free in motion. 



The close adhesion to it, in an embryonic manner, of its own splint, the squamosal, 

 (sq.), contributes much to this fixedness ; in the typical bird the squamosal becomes much 

 more mammalian, and after helping the prootic to form a glenoid cavity for the anterior 

 or outer head of the bone, then stands off from it to allow of great freedom of motion. 

 In all birds, as in Lizards, the large head of the quadrate bone reaches the cxoccipital, 

 passing over the lateral (tympanic) part of the opisthotic ; in the Chelonian this latter 

 part is so large tliat it receives the most backwardly projecting part of the condyle, and 

 excludes the exoccipital from the hinge ; still by far the greater part of the descending 

 plate of the investing mass, in the auditory region, goes to form the mandibular suspen- 

 sorium, whilst only a little of its hinder and inner part is devoted to the stapes, so con- 

 trary to what obtains in the ichthyic type. 



The proper positive explanation of the beanngs of these parts is not to be given by a 

 reference to the uses and fitnesses of them in their last or osseous stage, but by reference 

 to their primordial condition at a time when the thickening mass of cells around the 

 cephalic part of the notochord was growing equally over the simple (cutaneous) audi- 

 tory sac, and over the medulla oblongata, as the incipient occipital arch. At that time 

 this continuous tissue sent down, amongst others, the mandibular ray, the thick upper 

 part of which, as soon as chondrification commenced, appeared as a mass of cells, distinct 

 both from the side of the skull above and from the top of Meckel's cartilage below. 

 This great proximal expansion of the mandibular arch is a correlate of the arrested, 

 starved condition of its serial homologue next behind. In the cartilaginous stage of 

 the skull there was no definite boundary-line between the auditory capsule and the occi- 

 pital arch ; afterwards the bony pieces that result from the calcareous metamorphosis 

 of the cartilage in certain territories intercalate remarkably, and interchange not only 

 function but existence as we pass from group to group of the Vertebrata. Thus from 

 one common morphological stem there springs an endless variety of detail, — detail, 



