338 THE ANIMAL KINGDOM. 



sphenoid, the paired prae- and orbito-sphenoid ; the interorbital septum, 

 when it is present (Teleostei, Lacertilia, Aves) ; the paired lateral or ecto- 

 ethmoids in front of the orbits ; the internasal septum ; and sometimes a 

 prae-nasal rostrum, as in many Elasmobranchii, &c. The degree to which 

 the cranial cartilage extends dorsally varies. The part of the chondro- 

 cranium formed as above becomes (i) united with the cartilaginous capsules 

 of the nose, and ear, that of the eye remaining independent, and (2) con- 

 nected to visceral arches. The capsule of the nose is open externally and 

 coalesces with the ethmoidal region. The capsule of the ear is generally 

 closed, except where the auditory nerve enters it, and when two mem- 

 branous spots, the fenestrae ovalis and rotunda, are formed on its external 

 face. This capsule makes part of the lateral wall of the chondro-cranium 

 and is wedged in between the occipital and basi-sphenoidal regions. It 

 may even intervene in the floor of the skull between the two. The visceral 

 arches, which are independent of the chondro-cranium, are paired and post- 

 oral. The first is the mandibular or Meckel's arch, the second the hyoi- 

 dean, the third and succeeding are branchial arches. The hyoidean and 

 branchial arches are united ventrally by basal pieces or copulae, not, how- 

 ever, equal in number, to the arches themselves. Meckel's arch in some 

 Elasmobranchii and in Anura, among Amphibia, bends over the angle of 

 the mouth. The piece bent over lies in front of the mouth, and is cut 

 off, forming the palato-pterygo-quadrate bar, whilst the remaining part of 

 the arch forms the cartilaginous mandible. In other Vertebrata the 

 palato-pterygoid is an independent formation, and Meckel's arch is seg- 

 mented transversely, forming a small upper segment the quadrate or in 

 Teleostei and bony Ganoidei, the quadrato-metapterygoid, and a large lower 

 segment the cartilaginous mandible. The hyoidean arch also undergoes 

 transformation. Its upper part is cut off as a hyomandibular-symplectic in 

 Pisces, or a stapedial element in Amphibia and higher Vertebrata, while the 

 lower part of the arch forms the hyoid proper. This hyoid in the limited 

 sense, proceeding from its dorsal to its ventral end, is broken up typically 

 into stylo-, epi-, and cerato-hyal divisions, to which, in some Fish, is often 

 added a hypohyal. A branchial arch is typically divided in the same 

 manner, but the posterior arches in the series are simpler in structure x . 



With the addition of a variable number of labial cartilages round the 

 mouth, the above constitutes the cartilaginous skull. When it undergoes 

 a normal ossification the occipital region divides into a basi-, a right and 

 left ex-, and a dorsal supra-, occipital bone. The ear-capsule contains 



1 Dohrn has recently investigated the embryoes of various Elasmobranchii, and has come to the 

 conclusion that (i) the palato-pterygoid, (2) the mandible, (3) the spiracular cartilage, (4) the 

 hyomandibular, and (5) the hyoid represent as many visceral arches. With the exception of the 

 hyoid, these arches remain simple. The hyoid segments and its dorsal element fuses with the hyo- 

 mandibular in Selachoidei. The thyroid gland is, according to him, the remnant of the gill clefts 

 between the hyomandibular and hyoid arches. See Mitth. Zool. Stat. Naples, vi. 1885. 



