DEVELOPMENT OF THE SKULL IN THE CEOCODILIA. 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 Ichtliyopsidan parasphenoid 

 reappear as primarily distinct parostoses ; these bones are intimately connected with 

 the auditory apparatus. 



In the Crocodiles as well as in the Aves Katitae these basitemporals are lateral, 

 outside the basisphenoid ; but in the Aves Carinatae 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 (wliich start from the little cartilaginous 

 lingulse, 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 

 diploe 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 the 

 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 diploe. 



In the Crocodile, as pointed out by Professor Huxley, the tympanic cavity in the 

 quadrate communicates with a hollow in the "articulare" by the " siphonium." In 

 the Birds, as sho\^'n by Professor Nitzsch, the " siphonium " arises lehind 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 sucli centres 

 are seen inside the " cartilaginous annulus " in Bactylethra, an aglossal Anuran. 



VOL. XL— PAKT IX. No, 6. — October,li'b2>. 3 a 



