398 



curved plate of osseous matter, which appears on the outer side of 

 the proximal end of Meckel's cartilage, but is as completely indepen- 

 dent of it as is the ramus of the jaw of the rest of that cartilage. 

 In most birds it has no bony representative* . 



It is clear, then, as Professor Goodsirf has particularly stated, that 

 the os quadratum of the bird is the homologue of the incus of the 

 mammal, and has nothing to do with the tympanic bone ; while the 

 apparently missing malleus of the mammal is to be found in the 

 os articulare of the lower jaw of the bird. 



It would lead me too far were I to pursue the comparison of the 

 bird's skull with that of the mammal further. But sufficient has 

 been said, I trust, to prove that, so far as the cranium proper is con- 

 cerned, there is the most wonderful harmony in the structure of the 

 two, not a part existing in the one which is not readily discoverable 

 in the same position, and performing the same essential functions, in 

 the other. I have the more willingly occupied a considerable time 

 in the demonstration of this great fact, because it must be universally 

 admitted that the bones which I have termed petrous, squamosal, 

 mastoid, quadratum, articulare in the bird, are the homologues of 

 particular bones in other oviparous Vertebrata, and consequently, if 

 these determinations are correct in the bird, their extension to the 

 other Ovipard is a logical necessity. But the determination of these 

 bones throughout the vertebrate series is the keystone of every theory 

 of the skull it is the point upon which all further reasoning must 

 turn ; and therefore it is to them, in considering the skulls of the 

 other Ovipara, that I shall more particularly confine myself. 



Composition of the Skull of the Turtle. 



It has been seen that in birds the presphenoid, ethmoid, and or- 

 bitosphenoid regions are subject to singular irregularities in the mode 

 and extent of their ossification. In the turtle, not only are the 

 parts of the cranium which correspond with these bones unossified, 

 but its walls remain cartilaginous for a still greater extent. In 

 fact, if a vertical section be made through the longitudinal axis of a 

 turtle's skull, it will be observed that a comparatively small extent of 



* See Note III. 



f Reichert, however, had already clearly declared this important homology in 

 his ' Entwickelungsgeschichte des Kopfes,' p. 195. 



