AHREST OF DEVELOPMENT. 49 



ear-capsules is sometimes arrested. In the less marked forms 

 (agnathid) only the mandibular arch fails, but the posterior por- 

 tion of the cranial cavity is contracted. In the more marked 

 forms (synotia} the tympanic cavities coalesce, the hyoid arch is 

 constantly present, and the mandibular is always wanting. 



a. Agnathia. 



The brain-case is slightly contracted behind, especially im- 

 mediately in front of the ear- capsules, the mandible is absent. 

 The eyes and ears are approximated below, but the mouth and 

 pharyngeal cavities are continuous. 



192. The head of a foetal Pig with total deficiency of the mandi- 



ble. The eyes and ears are slightly approximated below. 

 Presented by R. Semple, Esq., 1820. 



193. A similar specimen. The roof of the skull has been re- 



moved to show the reduced size of the brain-case and 

 cranial axis behind. 



b. Synotia. 

 Syn. Monstres Otoc^phaliens, Geoff. 



The tympanic cavities are united below, the tympanic and squa- 

 mosal bones being fused together, although the periotic capsules 

 retain their normal position. This condition is explained by the 

 view that the tympanic and squamosal bones are integumental 

 ossifications, as has been pointed out by Mr. W. K. Parker. 

 Their position is most variable in malformations, whilst the 

 primitive ear always retains its normal relations with the axis. 

 The external ears are more or less fused together a little 

 above the hyoid bone ; there is usually only a single meatus 

 externus, and this communicates with the pharynx by an open 

 spiracle. The mouth is always completely separated from the 

 pharynx by the downward growth of the suborbital (maxil- 

 lary) arches. The lungs are commonly well formed, and are 

 inflated at birth by the passage of air through the spiracle or 

 spiracles, for there are two when the external meatus is double. 

 Sometimes the cranial and facial axes remain in their normal 

 relation with each other in animals, the cribriform plate of the 

 ethmoid remaining nearly vertical ; but sometimes the facial is 



E 



