190 ON THE MORPHOLOGICAL CONSTITUTION OF 



of the ethmoidal haemal arcli, or as in the laceitiaus aaaiust 

 the post-frontals. The squamous portion of the quadrate- 

 jugal bone is a mammalian siiperaddition, to adapt it to the 

 part it takes in the formation of the cranial wall. It is with- 

 drawn therefore from the quadrate or incudal portion of the 

 mandibular arch (which portion diminishes relatively), and 

 passes — as the entire bone does in the lizards — upwards to 

 be connected with the cranial wall. The development of the 

 first post-stomal visceral lamina in the bird appears therefore 

 to afford evidence that the mandibular haemal arch in the 

 bird and mammal includes a tympanic element, a quadrate 

 or incudal, a maUeal or articular, and the elements of the 

 corresponding side of the lower jaw. 



On the same grounds I am inclined to believe that the 

 articular piece of the lower jaw of the reptile and amphibian 

 is malleal like the corresponding piece in the bird, and not 

 the homologue of the condyle of the mammalian jaw. They 

 are all malleal portions of INIeckel's cartilage retained in con- 

 nection with the jaw. In like manner, I am inclined to 

 believe that the so-called tympanic bone of the reptile and 

 amphibian, like the quadrate or so-called tjrmpanic bone of the 

 bird, is not the homologue of the tympanic bone of the mam- 

 mal, but of the incus. The incus of the mammal has been set 

 free from its fundamental quadrate-jugal and pterygoid con- 

 nections to co-operate with the similarly released malleus in 

 the economy of the ear. The absence of proper tympanic 

 bones in the reptile and amphibian is explained by the absence 

 or feeble development of the tympanic cavity, I am inclined 

 to think, however, that traces of them may be detected under 

 and between the basi-sphenoid and occipital of the crocodile, in 

 the walls of those canals which connect, as Professor Owen has 

 shown, by a common tubular communication, the Sella Turcica 

 and the tympanic cavities with the basis of the cranium. 



The tympanic systems and lower jaw of the osseous fish 



