1. sphenethmoid. 2. palatine. 3. pterygoid. 4. quadrate. 5. columella. 6. exoccipital. 7. ventral cartilaginous wall of cranium. 8. pro-otic. 9. anterior fontanelle. 10. right posterior fontanelle. 11. quadratojugal. 12. nasal capsule. II. V. VI. IX. X. foramina for exit of cranial nerves.] The =auditory capsules= are fused with the sides of the posterior end of the cranium just in front of the exoccipitals. They are largely cartilaginous, but include in their anterior walls a pair of irregular cartilage bones, the =pro-otics= (figs. 24 and 25, 8). The cartilaginous area lying ventral to the pro-otic and external to the exoccipital is pierced by a rather prominent hole, the =fenestra ovalis=, which forms a communication between the internal ear cavity, and a space the tympanic cavity, which lies at the side of the head, and is bounded externally by the tympanic membrane. The fenestra ovalis is occupied by a minute cartilaginous structure, the =stapes=, and articulated partly to this and partly to a slight recess in the pro-otic is the =columella= (fig. 25, B, 5), a rod in part bony and in part cartilaginous, whose outer end is attached to the tympanic membrane. The columella and stapes are together homologous with the mammalian auditory ossicles and with the hyomandibular of Elasmobranchs. Sometimes the term columella is used to include the whole ossicular chain,--the columella together with the stapes. The =olfactory= or =nasal capsules= (fig. 25, B, 12) are fused with the anterior end of the cranium and differ from the auditory capsules in being to a great extent unossified. There are however two pairs of membrane bones developed in connection with them, the _vomers_ and the _nasals_. They are drawn out into three pairs of cartilaginous processes, on the dorsal surface into the =prenasal= and =alinasal= processes which bound the external nares, and on the ventral surface towards the middle line into the forwardly-projecting =rhinal= processes. The _nasals_ (figs. 24 and 26, 11) form a pair of triangular bones lying dorsolaterally in front of the fronto-parietals. Their bases are turned towards one another and their apices are directed outwards and backwards. They correspond in position with the prefrontals of the reptilian skull as well as with the nasals. The _vomers_ are a pair of irregular bones lying on the ventral surface of the olfactory capsules. Each bears on its inner and posterior angle a group of minute pointed teeth, while its outer border is drawn out into three or four small slightly diverging processes, the two posterior of which form the inner boundary of the =posterior nares= (fig. 24, B, 15). (3) THE JAWS. The =upper jaw= consists of a rod of cartilage connected with the cranium near its two ends, but widely separated from it in the middle. It is almost completely overlain by membrane bone. With its posterior end the lower jaw articulates.