356 



PALAEONTOLOGY OF OHIO. 



1868, and has since been occasionally employed by authors under the 

 name of Labyrinthodontia (Owen). As then and now adopted, its scope 

 is much more extended than that of the order proposed by Prof. Owen 

 under the latter name, including as it does his order Ganocephala with 

 it, as well as numerous forms discovered subsequent to Prof. Owen's class- 

 ification, both in England and North America, to some of which thp 

 term Microsauria has been applied. As the labyrinthic character of the 

 teeth does not appear in a number of the latter forms, and as the order 

 as now defined by me is established on characters in large part not used 

 by Prof. Owen, I retain the name given in 1868. It is probable that the 

 Labyrinthodontia will be found to be a definable subdivision of this order, 

 but with its characters we are as yet in a measure unacquainted. The 

 nearest approach to the true Labyrinthodons seen in any American genus 

 is found in the Leptophractus of the present essay. 



II. THE CLASSIFICATION OF THE STEGOCEPHALI. 



The classification of the Stegocephali presents many diificulties, which 

 are mainly due to the character of the skeletons of these animals. 

 Many of the bones are thin, and the sutures often squamosal. T^e com- 

 position of the skull is, on these accounts, often difficult to determine, 

 especially as the specimens are generally injured by pressure, etc. I 

 formerly arranged these animals in four orders, adopting from Prof. Owen 

 the divisions Labyrinthodontia and Ganocephala, employing Prof. Dawson's 

 term Microsauria, with modified definition, and adding a new one, the 

 Xenorhachia. These divisions were based on characters mostly already 

 in use, namely, the condition of the vertebral column as to ossification, 

 the presence or absence of branchial bones, and the presence or absence 

 of inflections of the enamel of the teeth. These characters I do not now 

 regard as definitive of divisions of high grade, for the following reasons : 

 There are genera, otherwise nearly allied, which differ in the degree of 

 ossification of their vertebral columns, e. g., Sauroplew-a and Colosteus, 

 Pelion and Amphibamus. It is uncertain whether all parts of the column 

 are similarly ossified in the same species, e. g., Pelion Lyellii. The pres- 

 ence of branchial hyal bones does not characterize genera with unossified 

 vertebral column only, as Archegosaurus, but is evident in some of the 

 osseous types, e. g., Oestocephalus. The inflection of the investing enamel 

 of the teeth, characteristic of Labyrinthodon, Owen, is well marked in 

 Leptophractus of the Ohio fauna, but appears to be wanting in the teeth 

 of some of the small species of other genera. Yet I am by no means 

 sure that this is the case, as I observe it very clearly in the small species 

 of Ptyonius. I do not find it well marked in Oestocephalus, and believe 



