THE BATRACHIA OF NORTH AMERICA. 15 



As regards the connection of the class, as a whole, with other classes 

 of vertebrata, it is very probable that the extinct orders, as the Gano- 

 cephala, were derived from some extinct form of Dipnoan fishes, more 

 or less related to the group of which the genus Ceratodus is a represent- 

 ative. In this type we have a persistent chorda dorsalis, fins which 

 present the type from which ambulatory limbs were derived, a pelvis, 

 and a cranium nearer that of the batrachians than most other fishes 

 present. The Crossopterygia are a little on one side of the parental 

 stem, since they have no pelvis, and their limbs begin to show a begin- 

 ning of that reduction and specialization which is carried to such an 

 extent in the Actinopterygia, or typical fishes. 



From the Ganocephala we derive the remaining orders, all of which 

 possess two occipital condyles. The intercentra, which are small in the 

 prepelvic vertebral axis of the Ganocephala, assume a large development 

 in the Embolomeri, which thus have two bodies to each neural arch 

 throughout the series. It is probable that the reptiles took their origin 

 from this group by the gradual reduction of the intercentra, and the 

 continued dominance of the centra. It is possible also that the Em- 

 bolomeri possess but one occipital condyle, which, uniting with an iu- 

 tercentrum, formed the single occipital condyle of the Reptilia. 



In the direction of the Rbachitomi there is reason to believe that the 

 intercentra became predominant in the vertebral axis, and that the 

 centra soon disappeared. This order gave origin to the Stegocephala, 

 in which the centra are wanting; and this order was the source of the 

 Proteida on one hand, and the Salientia on the other. The former,of 

 all the existing orders is the only one which retains the os intercalare 

 of the Palaeozoic types. From the Proteida we get the Urodela, and 

 from the latter the Trachystomata, as will be more especially shown 

 under that order. 



The Ganocephala (Trimerorhachis) and the Ehachitomi (Zatrachys) 

 had a well developed columella auris, which extended from the fenestra 

 ovale and turned upwards and backwards to the notch of the posterior 

 outline of the skull between the os intercalare and the base of the quad- 

 rate. It is highly probable that this notch was occupied by a tympanic 

 drum.^ In the Proteida, Urodela, and Trachystomata, there is no col- 

 umella. In the Salientia there is a chain consisting of one bone and 

 two cartilages extending between the stapes and the membranum tym- 

 pani. Thus the Urodela in this respect have undergone degeneration, 

 while the Salientia have undergone specialization. With regard to 

 other parts of the skeleton all the later and recent orders must be re- 

 garded as having undergone' degeneration, in view of the extensive loss 

 of parts. {See Origin of the Fittest, by E. D. Cope, On the Evolution 

 of the Vertebrata, progressive and retrogressive.) 



1 See Cope On the ossicula auditus of the Batrachia, Amer. Naturalist, 1868, p. 464 ; 

 Journal of Morphology, November, 1888. 



