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Show 214 ON THE STRUCTURE OF THE SKULL. 8.1 ·a th Am11hibia are devoid of Like the Carp and the 1 urm s, ~ :r . . f 1 bl an inter-o~bital septum, the cranial cavity remai~Ing o t~ er~ y Y . h . 't 1 £ ramen to its antenor terminatiOn. even size from t e ocmpi a o . 1 fl F (F. BB) the skull is roofed 1n by two ong at In the rog Ig. · h · 1 membrane b ones (pa , Fr) ' which correspond w1th. t . e paneta s and f ron t a 1s , an d , I·n fact ' each originate in two distinct centres, Fig. 86. Pm~ 0 A f: b B f·om Lelow · C from the Fig. 86.-Skull of Rana esculenta. Seen ·' rom a ove .: ·dl' ~ . the :, temporo-side (after Duges).-x, the parasphenmd j y, the gn c one, z, mastoid" of Duges. one in front and one behind. In front of these a~e two .other membrane bones (Na, Na), which have been varwusly Interpreted but which probably answer to the nasals. On the base of the 'skull is the long single parasphenoid (x), the hinder p~rt of which is produced into two broad l~teral processes, whwh underlie the auditory capsules. b When these membrane bones have been stn·p ped of f' a su. - jacent cart1.l ag1. nous crani· um b ecomes appar ent, produced beh. md into two lateral enlargements, or tuberosities, for the audito~y organs, and having certain fontanelles or membranous ~pac.es: its upper wall (Fig. 87). In the substance of this cartda~mo f cran1. um, posten.o rly, are two oss1' £ cat I' ons, on e on each s1de o 'l'HE SltULLS 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 audit~lry organ. Tho front and upper wall of each auditory tuberosity is also largely ossified, th~ 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 a_nd of Man. A fifth ossification of the cartilage is the very singular bone (y) which Cuvier termed the os en ceint-ure, 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 fullv 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 perfo1;ates that bone, probably represents 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 mo that the common Toad has a tllin houv crest answer-ing to tho epiotic. • |