302 PROF. W. K. PARKER ON THE STRUCTURE 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 Dactylethra and in 
some of the Phaneroglossa (e. g. Caliula and Hylaplesia) the columella is as large as 
the average “ hyomandibular ” of Fishes ; in Pipa the columella is formed and finished 
much earlier than in the other types. 
In the Ophidia the 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 Chameleonide 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 avian, 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-drum ; 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 
