THE AMPHIBIAN SKELETON. 175 



long tails. But in the Batrachia, the total uiimber of verte- 

 brse does not exceed eleven, of which eight belong to the pre- 

 sacral region, one to the sacrum, and two (modified vertebrse) 

 to the coccygeal region. The transverse processes of some of 

 the presacral vertebrse are usually very long, but there are 

 no separately ossified ribs. The transverse processes of the 

 sacral vertebra are very large and expanded, and its centrum 

 has usually a single concavity in front and a double convexity 

 behind. 



The coccyx consists of a long, cylindroidal, basal bone 

 proceeding from the ossification of the sheath of the termi- 

 nation of the notochord, and corresponding with the urostyle 

 of the Teleostei ; and of two neiiral arches, which lie over its 

 anterior end, and become ankylosed with it. The anterior 

 face of the coccyx usually presents two concave facets for 

 articulation vsdth the posterior convexities of the sacrum. 



The cavity of the cranium is not naiTowed anteriorly by 

 the development of an interorbital septum in any Amphi- 

 bian. All existing Amphibia have ex-occipitals developed 

 in the walls of the cartilaginous cranium; but it is not 

 certain that any such ossifications existed in Archegosaurus, 

 though they are present in other Labyi-inthodonts. 



No Amphibian possesses a complete basi-occipital, supra- 

 occipital, basisphenoid, alisphenoid, or presphenoid car- 

 tilage bone. In existing Amphibia, a pro-otic ossification 

 appears to be very constant. The constant existence of 

 distinct opisthotic and epiotic elements is doubtful. 



The Frog's skull is characterised by the development 

 of a very singular cartilage bone, called by Cuvier the " os 

 en ceinture," or girdle-bone. This is an ossification which 

 invades the whole circumference of the cranium in the 

 presphenoidal and ethmoidal regions, and eventually as- 

 sumes somewhat the form of a dice-box, with one-half of its 

 cavity divided by a longitudinal partition. The latter, cor- 

 responding with 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 nerve. The septum, therefore, answers 



