188 ON THE MORPHOLOGICAL CONSTITUTION OF 



jaw. It then extends inwards, so as to form the floor of the 

 tympanum and Eustachian tube, folding up before and be- 

 hind, and thus, inclosing the incus and malleus, leaves the 

 latter connected to the jaw through the tympanic fissure. 



The development of the first post-stomal visceral lamina 

 appears, therefore, to indicate at least four elements on each 

 side of the mandibular haemal arch in the mammal. The 

 tympanic element is probably complex ; the mandibular con- 

 sists of at least two portions, one on the outer, the other on the 

 inner side of the corresponding portion of Meckel's cartilage. 



In the bird the omoid and palate bones are formed like 

 the pterygoid and palate bones of the mammal, in the so- 

 called " superior maxillary process." In the proper first post- 

 stomal visceral lamina, the primordial cartilaginous streak 

 divides, as in the mammal, into two portions. The upper 

 and smaller of the two becomes the quadrate bone ; the 

 lower and longer portion Meckel's cartilage becomes en- 

 veloped in the corresponding half of the lower jaw ; but 

 instead of the upper end of this portion forming the slender 

 process of a malleus, it remains as the peculiarly-formed 

 articular piece of the jaw itself. The original intimate con- 

 nection of the rudiment of the pterygoid bone, in the so-called 

 " superior maxillary process," with the upper or incudal por- 

 tion of the primordial cartilaginous streak of the first post- 

 stomal visceral lamina of the mammal, speedily diminishes ; 

 but in the bird, not only does the pterygoid or omoid bone 

 rapidly increase in relative size and configuration ; but the 

 quadrate portion of the first visceral streak does so likewise. 

 The latter also exhibits, attached to its outer side, as the 

 omoid is to its internal process, a styliform ossicle, the rudi- 

 ment of the quadrate-jugal bone, which again is connected 

 anteriorly to the jugal. 



Eeichert, who has minutely described and figured the 

 development of this visceral lamina in the bird, makes no 



