THE SKULLS OF AMPHIBIA. 215 



the occipital foramen, which nearly meet in the middle line 

 above and below. These, the ex-occipitals, bear the condyles 

 for articulation with the atlas, and partly shelter the posterior 

 portion of the auditory organ. The front and upper wall of 

 each auditory tuberosity is also largely ossified, the resulting 

 bone protecting the anterior part of the organ of hearing, and 

 being perforated, or notched, for the transmission of the third 

 division of the trigeminal. This therefore is, without a question, 

 the homologue of the pro-otic bone of the fish and of Man. 



A fifth ossification of the cartilage is the very singular 

 bone (i/) which Cuvier termed the os en ceinture, or " girdle bone," 

 from its encircling the anterior part of the cranial cavity. This 

 bone has somewhat the form of a dice-box, with one end divided 

 by a longitudinal partition. The latter — the front part of the bone 

 — extends into the prefrontal processes in some frogs, protects 

 the hinder ends of the olfactory sacs, and is perforated by the 

 nasal division of the fifth. The median partition therefore must 

 answer to some extent to the ethmoidal septum, while the lateral 

 parts of the anterior division of the bone correspond with the 

 prefrontals. On the other hand, the hinder division of the bone 

 is an ossification of each wall of the cranium, in front of the exit 

 of the optic nerves ; so that I conceive this part of the bone 

 can only answer to the orbito-sphenoids, united above and below. 

 Upon this view of its nature, the girdle bone answers to at least 

 five bones, viz., the ethmoid, prefrontals, and orbito-sphenoids. 



No alisphenoid is developed in any Amphibian. There is 

 no separate opisthotic in the adult state, and I am not fully 

 satisfied as to the existence of any distinct epiotic, though such 

 a bone has been affirmed to exist (under the name of "mastoid") 

 in the axolotl and the Menobranchus* 



The anterior part of the ex-occipital, in front of the foramen 

 for the eighth nerve, which perforates that bone, probably re- 

 presents the opisthotic, as between it and the posterior external 

 margin of the pro- otic is placed the fenestra ovalis, a structure 

 not met with in the class Pisces. 



The facial bones are, for the most part, readily determinable ; 



* Mr. Parker informs me that the comuio:i Toad has a thin bony crest answer- 

 ing to the epiotic. 



