38 Mr. H. G. Seeley on the Origin 



With the termination of branchial respiration (and the 

 branchial arches appear to represent the epipleural elements 

 of cervical ribs) the function of the pleural element of the first 

 cerebral arch appears to cease, and the bones of the operculum 

 are no longer developed ; and in the same way, when the 

 respiratory function becomes changed, so that the animal 

 breathes by lungs, the branchial bones are merged in the 

 hyoid ; the hyoid loses its heavy osseous character, and has a 

 less firm attacliment to the hyomandibular. This bone then 

 gives attachment to the quadrate, and becomes the main sup- 

 port for the mandible ; so that it appears to be the bone which 

 among the higher Vertebrata is named the squamosal. In 

 the fish there are bones in front of the quadrate bone which 

 are called metapterygoid and symplectic. I have doubted 

 whether these bones may not have originally stood in the 

 same relation to the second visceral arch which the hyo- 

 mandibular held for the first, since they persist, the meta- 

 pterygoid becoming the quadrato-jugal, and the symplectic 

 becoming the supraquadrate ; and they both appear ultimately 

 to be absorbed into the squamosal. If this view were taken, 

 it would in no way be inconsistent with fact, and would only 

 show that the lower jaw had been carried a stage backward, 

 while it would explain the existence of two otherwise obscure 

 bones, and justify their disappearance under the influence of 

 potential growth in those animals in which they are wanting, 

 since in the Amphibia is seen a similar lateral joining-up and 

 absorption of the branchial arches into the hyoid. 



Already it has been remarked Ihat the lower jaw always 

 articulates with the squamosal bone, the squamosal bone 

 being, as we have just seen, apparently the proximal element 

 of a visceral arch. Sometimes the squamosal bone itself is 

 free, as in serpents ; but usually it is firmly fixed in the skull. 

 Sometimes, also, the quadrate bone is firmly wedged in the 

 skull, as in Crocodiles, Chelonians, Hatteria^ and most of the 

 extinct Monocondylia ; but there is no' evidence whatever of 

 any other part of the lower jaw (as the os articulare) being- 

 united with the skull> And in all those animals in which the 

 quadrate bone is joined with the skull, the lower jaw remains 

 composite. In the highest Monocondylia (birds) the quadrate 

 bone remains distinct, while the squamosal bone has entered 

 into the skull in the same way as in mammals, and furnishes 

 a concave articulation for the quadrate bone exactly like that 

 which in mammals is given to the lower jaw. Now, in so far 

 as the lower jaw occupies the position of a rib, the influence 

 of potential growth upon it would be to make it ever more 

 and more like a rib in simplicity of structure : hence I pre- 



