DEVELOPMENT OF THE SKULL IN THE CROCODILIA. 303 
stapedial plate has only a temporary and imperfectly separate existence; here, however, 
the cartilaginous “ annulus tympanum” reappears—a large and highly developed “ spi- 
racular cartilage.” 
But the Crocodiles and Birds have the most remarkable development of the tympanic 
labyrinth ; and in them the two basitemporal wings of the Ichthyopsidan parasphenoid 
reappear as primarily distinct parostoses; these bones are intimately connected with 
the auditory apparatus. 
In the Crocodiles as well as in the Aves Ratite these basitemporals are lateral, 
outside the basisphenoid; but in the Aves Carinate they are much larger, and meet 
and coalesce below the skull-base. 
In the Bird the columella is a pharyngo-hyal, with a dilated upper part; it coalesces 
with an epihyal (=stylohyal) rudiment of the main bar, through the medium of an 
“infrastapedial” (=interhyal) tract, which is later in appearance than the other 
parts. The distal part of the hyoid arch is a hypohyal, which meets its fellow at a 
sharp angle in the tongue to form a “ glossohyal.” 
Here we miss what is found in the Crocodile, namely, a distinct suprastapedial; the 
distal rudiment of the main bar does run into the ceratohyal region for a small extent. 
In the Birds the Eustachian tubes open at the mid line in one common vestibule, which 
is the homologue of the middle Eustachian passage of the Crocodile. Also in Birds 
the periosteal growths of the basisphenoid (which start from the little cartilaginous 
lingul, parts present in both Crocodiles and Birds) there form, above the basitemporal 
floor, a pair of “anterior tympanic recesses.” These trumpet-shaped cavities answer, 
in some degree, to the passages in the Crocodile where the lateral and median Eusta- 
chian tubes combine; they converge towards each other, but do not meet, in the thick 
diploé of that part of the skull. 
In the Crocodile the quadrate forms much of the tympanic cavity; in the Bird it is 
pneumatic, and opens by a hole into that cavity, which is enlarged by a wing of tlie 
exoccipital. That cavity also, as in Crocodiles, communicates with cavities in the 
occipito-otic bones above. In the Crocodile the whole hind skull is excavated by these 
pneumatic diverticula; in Birds the whole hind (as well as fore) skull is pneumatic, 
but the cavities are traversed by fine reticulations of the diploé. 
In the Crocodile, as pointed out by Professor Huxley, the tympanic cavity mm the 
quadrate communicates with a hollow in the “articulare” by the “siphonium.” In 
the Birds, as shown by Professor Nitzsch, the “‘siphonium” arises behind the quadrate 
in the general tympanic space. 
In the Crocodiles I have seen no bony centres round the “siphonium ;” but in the 
Birds these fragments of the “os tympanicum ” (proper) sometimes number as many as 
six or seven, and the main bone forms a ring to the pneumatic tube; two such centres 
are seen inside the “ cartilaginous annulus” in Dactylethra, an aglossal Anuran., 
VOL. XI.-—PART Ix. No. 6.—October, 1883. 5A 
