302 PEOF. W. K. PAEKEE ON THE STEtJCTUEE AND 



During the middle period of incubation the hyostapedial chain becomes continuous, 

 and remains for a time united with the articular part of the mandible ; and the air-cell 

 in that part, which was first seen in the last stage, burrows still deeper in the mandible 

 at this time. A large annular (spiracular) cartilage is to be seen now, as in the Chelonia 

 and Batrachia. 



In the beginning of the latter period of incubation the endoskeletal bony centres 

 are found; the tympanic labyrinth is rapidly developed, and the hyoid arch first 

 becomes severed from the mandible, and then breaks up again into its primary 

 segments. 



The specialization of the first pair of clefts in relation to the organ of hearing in the 

 Crocodile is so great and so remarkable, that it is worth while to compare it with what 

 is seen in other types. 



In some of the Urodeles (where the stapes is first seen) a second pharyngo-hyal 

 segment is seized by the outgrowing ectosteal plate of the stapes, and is thus united 

 with it to form the columella ; in that group there is no cavum tympani. 



In the majority of the Anura the first cleft, which in them never opens externally, 

 becomes a considerable tympanic cavity, and opens by a large lateral, internal hole 

 between the pedicle and the stylohyal. The stapes is always distinct, even when, as 

 in Bombinator, there is no columella and the merest trace of a tympano-Eustachian 

 pouch ; where, as in most cases, the columella is present, it appears long after the 

 stapes, and may be composed of one, two, or three segments ; it begins as a styliform 

 second pharyngo-hyal. The epihyal ( = stylohyal) end of the suddenly elongated hyoid 

 bar, may, as is the rule, coalesce with the auditory capsule behind the Eustachian 

 opening, or be attached by ligament, or coalesce with the second or permanent pedicle ; 

 in all these cases it is below and a little behind the emerging facial nerve. In the 

 Aglossal Anura the Eustachian openings meet at the mid line ; in Dacti/let/ira and in 

 some of the Phaneroglossa (e. g. Callula and Ili/lajj/esia) the columella is as large as 

 the average " hyomandibular " of Fishes ; in Fqia the columella is formed and finished 

 much earlier than in the other types. 



In the Ophidia tlie columella is formed by ossification of a single pharyngohyal ; but 

 there is no cavum tympani, and, therefore, no pneumatic bones. 



In the lower Lacertilia (e. g. Hatteria) and the Chamseleonidse the cavum tympani is 

 scarcely at all developed, and where it is, as in most of the types, there are no pneumatic 

 bones ; yet the columella is very a\ ian, and has, besides its bony shaft and base, upper, 

 external, and descending cartilaginous processes. 



In the Chelonia, especially in the lesser freshwater kinds, the squamosal and the 

 quadrate bones are hollowed out to form a large and elegant ear-dium ; the opisthotic 

 (mastoid) is also excavated behind the main cavity. 



Their columella, with its discoid " extrastapedial," is very much like that of several 

 Anura, especially that of the "Aglossa," and for a time is in two segments; the oval 



