that it encircles tlie confluent opening of the optic foramen, 

 orbital fissure, and foramen rotundum (the optic-orbital- 

 round fissure). The palatine is penetrated by a large 

 sphenopalatine foramen, which acts also as the orbital por- 

 tal of the palatine canal. This canal has several palatal 

 openings posterior to the vertical of the orbital opening. 

 Posterior to the sphenopalatine opening, the orbital wing is 

 pierced by the pterygoid canal. 



The frontal is small and forms much of the medial wall 

 of the orbit. It is pierced at the upper orbit margin by an 

 orbitonasal foramen. The parietals are large and form much 

 of the roof of the cranium. Below the parietal is the large 

 cranial-wall wing of the orbitosphenoid extending forward 

 and down to meet and fuse with its opposite along the mid- 

 line from below the ethmoid foramen ventroposteriorly to 

 between the optic-orbital-round fissures. 



Behind the orbitosphenoid is the large dermal plate of 

 the petrosal. Between the ventroanterior margin of the 

 petrosal, the posterior margin of the palatine, and above 

 the anterior end of the ectopterygoid, there is a small ex- 

 posure of the basisphenoid — again usually identified as the 

 alisphenoid. This basipterygoid process extends outward 

 and slightly downward and is not pierced by a canal. Be- 

 hind this process, between the petrosal and the ectoptery- 

 goid, is a large foramen for the mandibular branch of the 

 trigeminal nerve. This is an analog of the foramen ovale. Its 

 position behind the basisphenoid wing is used to support 

 the contention that the wing is a vestigial alisphenoid. It 

 seems best to consider this process a part of the basisphenoid, 

 identifiable as the basipterygoid process. 



A long narrow temporal canal passes between the tem- 

 poral fossa, inside the cover of the squamosal, and the small 

 posttemporal fossa, which is no more than a foramen at the 

 posterior margin of the squamosal. The squamosal lacks a 

 distinct articular area for the mandible. 



The squamosal and maxilla have long zygomatic proc- 

 esses, which form the zygomatic arch. There is no jugal 

 bone. The carotid canal passes up and forward from the 

 posterolateral angle of the basisphenoid to its foramen in 

 the lateral wall of the sella. 



The petrosal bones, at either side of the basioccipital, 

 lie above a distinctly triangular tympanic fossa. Below the 

 fossa lies the tympanic bone, to which is fused the anterior 

 process of the malleus. The incus is fused to the malleus. 

 The fossa is margined behind by a transverse mastoid proc- 

 ess, laterally by the squamosal, and anteromedially by a 

 ridge of the dermal part of the petrosal that extends to the 

 outer margin of the ectopterygoid and then above that 

 bone, along with a corresponding elevation of the ectoptery- 

 goid. The posterior end of the ectopterygoid is separated 

 from the petrosal by a gap. This gap is said to transmit 

 blood vessels; the eustachian tube passes lateral to the tip of 

 the ectopterygoid. 



The tympanic fossa opens posterolaterally through a 

 primitive stylomastoid foramen between the lateral, but very 



small, tympanohyal process and the medial mastoid process. 



At the anterior end of the fossa there is an anterior lacerate 

 fissure between the vestibular portion and the dermal wing 

 of the petrosal. This fissure passes forward and upward into 

 the cranial cavity. In the posterolateral corner of the fossa is 

 the round fenestra vestibuli, into which fits the columelliform 

 stapes. Above, lateral and anterior to this, is the common 

 opening of the facial canal and the vena capitis lateralis 

 canal. 



The internal view of the skull (Figure 3-10 A,D) shows a 

 cribriform plate complete with nasal septum and turbinals. 

 The sella has well-developed lateral margins formed by the 

 clinoid processes and a low dorsum sellae. The carotid canal 

 opens through the lateral wall of the sella near its posterior 

 end. The anterior lacerate fissure has its internal opening 

 just lateral to the dorsum sellae. There is no subarcuate 

 fossa but otherwi.se the foramina of the petrosal are typical. 

 A perilymphatic foramen has not been identified but is 

 probably present in the margin of the petrosal adjacent to 

 the internal portal of the jugular foramen. 



The mandible is highly modified; there are weak internal 

 and external processes posteriorly. The foramina are like 

 those of the placental. 



The adult hyoid (Figure 3-11 D) appears to be like that of 

 the platypus except that only the hypohyal is ossified in the 

 hyoid arch. 



The monotreme head skeleton is unique, as compared 

 with those of the other mammal groups, in several features: 

 lack of an identifiable alisphenoid and replacement of this 

 bone by a dermal wing of the petrosal, presence of both a 

 pterygoid and ectopterygoid which do not lie in contact, 

 the position of the pterygoid in the dorsolateral wall of the 

 nasal passage, the presence of a temporal canal passing be- 

 tween the cranial wall and the squamosal, the squamosal not 

 forming part of the cranial wall, the absence of ajugal, and 

 the presence of ceratobranchials II and III. 



In addition to these characteristics there are other minor 

 features, such as lack of defined incisive foramina (or palatal 

 processes of the premaxillae), and the presence of external 

 and internal processes on the mandible. 



On the whole the features of this type of head skeleton, 

 along with its lack of teeth, the modification of the snout, 

 and extension of the secondary palate far back, suggest 

 specialization more than retention of primitive features. The 

 lack of a bulla and the passage of the vena capitis lateralis 

 through the tympanic fossa are primitive. 



DEVELOPMENT OF THE HEAD SKELETON 



Chondrocranlum 



The chondrocranlum can be defined as the cartilaginous 

 forerunner of the adult endocranium (enclosing the brain, 

 inner ear, and nasal passages and forming the socket for the 

 eye) and of the visceral skeleton. It varies in form as it 

 changes its function at different ages of the animal. In the 



DEVELOPMENT OF THE HEAD SKELETON • 49 



