132 VERTEBRATE SKELETON 



The pterygoquadratc arises apart from the chondrocranium, but soon 

 becomes connected with it by an epipterygoid bar behind, and in front by fusion 

 a Kttle behind the nasal capsule. It rapidly increases in size, extending forwards 

 nearly to the suprarostral, the small Meckel's cartilage being articulated to it 

 far forwards, and coming into close relations with a small infrarostral cartilage 

 (probably a lower labial), which supports the horny lower jaw. At metamor- 

 phosis the articulation of the lower jaw shifts (in a way not easily described) 

 from near the anterior end of the pterygoquadrate to a point behind the 

 epipterygoid, thus greatly increasing the gape of the mouth. During metamor- 

 phosis the infrarostrals join the MeckeHans, eventually ossifying as a mento- 

 meckelian on either side. The suprarostrals are lost. A cartilage tympanic 

 annulus to support the tympanic membrane becomes closely related to the 

 lateral side of the tympanum. 



The cartilage bones of the adult include the pairs each of exoccipi- 

 tals, prootics, stapedial structures, ethmoids (paired in ossification) 

 and the articulare parts of the pterygoquadrate. (In some of these 

 ossification is not complete.) The ethmoid (sphenethmoid, ah- 

 sphenoid, os en ceinture, 'girdle bone') begins as a pair of ossifica- 

 tions close to the olfactory foramen and never extends back so as to 

 include the optic nerve, excluding an alisphenoid homology. The 

 prootics ossify in the anterior cupula and extend thence into basal 

 and trabecular cartilages as well as to the other sides of the otic 

 capsules. The exoccipitals are separate and always bear a pair of 

 occipital condyles. 



Each parietal, which covers part of the otic capsule, fuses early 

 with the frontal of the same side to a fr onto -parietal bone which 

 reaches to the ethmoid, while behind it meets the exoccipital. The 

 nasal Hes obliquely over the nasal capsule, a gap existing between it 

 and the ethmoid behind and the premaxilla in front. No prefrontal 

 occurs. A septomaxillary develops in the nasal region behind the 

 naris and covers the opening of the lacrimal duct, a part of which is 

 more or less completely enclosed in the bone. The squamosal 

 (separated from the parietal by the prootic which appears on the 

 surface of the skull) is three-branched, the lateral ramus entering the 

 hinge of the jaw, the anterior extending forwards above the pterygo- 

 quadrate (subocular) bar and in some genera may meet the maxilla. 

 The slender maxilla connects behind with a bone (usually called the 

 quadratojugal, but as it includes material derived from the pterygo- 

 quadrate has been called a quadratomaxillary) completing an arch 

 like the zygomatic of other Vertebrates, although it contains no 

 zygomatic bone. 



