PHYLUM CHORDATA 



361 



relations to the brain-case, the basi-occipital (b.o.) as in the Lizard, 

 bearing the occipital condyle. The basi-sphenoid (Fig. 972, B. SPH) 

 is a large bone forming the greater part of the basis cranii, and 

 continued forwards, as in the Lizard, by a slender rostrum (Fig. 971, 

 pa.s., Fig. 972, RST.\ which represents the anterior portion of the 

 para-sphenoid. On the ventral aspect of the basi-sphenoid paired 

 membrane bones, the basi-temporals (Fig. 972, B. TMP) are deve- 

 loped, and become firmly ankylosed to it in the adult : they pro- 

 bably represent the posterior portion of the para-sphenoid. The 

 tympanic cavity is bounded by the squamosal (Fig. 971, sq.), 

 which is firmly united to the other cranial bones. The main part 



ORB.SPf 



FIG. 972. Sagittal section of a Bird's skull (diagrammatic). Cartitac/e bones AIi.SFH.alisphe- 

 noid ; ART. articular; B.OC. basi-occipital; EP.OT. epiotic ; EX. OC. ex-occipital ; 

 M.ETH. mesethmoid ; OP.OT. opisthotic ; ORB.SFH. orbito-sphenoid ; PR.OT. pro- 

 otic ; QU. quadrate; S. OC. supra-occipital. Membrane bones ANG. angular; B. TMP. 

 basi-temporal ; COR. coronary; DNT. dentary ; FR. frontal; JU. jugal; LCR. lacrymal; MX. 

 maxilla; NA. nasal; PA. palatine; PMX. pre-maxilla ; PTG. pterygoid ; QU. JU. quadrato- 

 jugal; RST. rostrum; S. ANG. supra-angular; SPL. splenial ; SQ. squamosal; VO. vomer; 

 flc. fos. floccular fossa ; mx. pat. pr. maxillo-palatine process ; opt. for. optic foramen ; orb. pr. 

 orbital process ; ot. pr. otic process ; pty.fos. pituitary fossa. 



of the auditory capsule is ossified by a large pro-otic (Fig. 972, 

 PR. OT) : the small opisthotic of the embryo early unites with the 

 exoccipital, the epiotic with the supra-occipital. The presphenoid 

 and mesethmoid together form the interarbttal septum (Fig. 971, 

 i.o.s.), a vertical partition, partly bony, partly cartilaginous, which 

 separates the orbits from one another. It is very characteristic of 

 the Bird's skull that the immense size of the eyes has produced 

 a compression of this region of the skull. The ectoethmoids or 

 turbinals are comparatively poorly developed, in correspondence 

 with the small size of the olfactory organs. There are large 

 lacrymals (Fig. 971, lc. t Fig. 972, LCE.) and the nasals (na, na', 

 na"; NA) are forked bones each furnishing both an inner and an 

 outer boundary to the corresponding nostril. 



