EXTINCT BATRACHIA. 357 



that it may bo found in various degrees of development in other genera, 

 so as to be practically useless in discrimination of groups of high rank, 

 at least until we are able to give it a more thorough investigation. 



A number of groups maj be discerned into which the genera fall, and 

 which mostly present tangible structural characters.* The presence or 

 absence of ribs is an important consideration. The presence or absence 

 of limbs can generally be determined, although in some genera, as 

 Ptyonius, they are very small. The extent of the vertebral column de- 

 termines the habit of the animal in a marked manner. Two types of 

 cranial form maj^ be recognized; in the one it is lanciform, and composed 

 of attenuated bones in which the sculpture is but little marked; in the 

 other the skull is broad, obtuse, and flat, and strongly sculptured. The mere 

 difference in the outline form of the head does not indicate family char- 

 acters by itself, but in the present case each type is associated with other 

 features, and future investigation will doubtless add to their weight. 

 The divisions thus distinguished are the following, and I -believe that 

 they may be accorded family rank : 



Family I. PHLEGETHONTiiDyE. Vertebral column very elongate, and with- 

 out ribs ; limbs probably wanting ; cranium lanciform, of light construc- 

 tion. Genus Phlegethontia. 



Family II. Molgopiiid^. Vertebral column elongate, furnished with 

 ribs; limbs ? wanting; cranium unknown. Genus Molgophis. 



Family III. Ptyoniid.e. Form moderately elongate; ribs present; limbs 

 present, weak ; cranium light, lanciform ; belly protected by osseous 

 rods. Genera, Oestorcphalus, Ptyonius, Hyphasma, Lepterpeton (European). 



Family IV. Tuditanid.e. Form lizard-like; limbs well developed; cra- 

 nium broad and obtuse. Genera, Tuditanus, Dendrerpeton, Hylonomus, 

 Leptophr actus, Ceraterpeton, ? Pelion. 



Family V. CoLOSTEiDyE. Form lizard-like; limbs present; belly covered 

 with rhombic scutes; vertebrae often unossified. Genera, Sauropleura, 

 Colosteus, Amphibanius. 



There are several genera whose position in relation to the above fam- 

 ilies can not be definitely determined at present. Such are Pleuroptyx, 

 Brachydides, Thyrsidivm, and Eurythorax, in all of which some of the 

 essential parts are wanting. The genus Cocytinus, so far as is ascertain- 

 able from a view of the inferior side of the skull, does not belong to the 

 Stegocephali, but more probably to the Proteida. 



With regard to generic characters, there are minor variations observ- 



* See Transactions American Philosophical Society, April, 1874, where these are 

 pointed out. 



