The order is suggested in the Coal Measures by a single species, known from a single poorly preserved specimen (plate 24, fig. 1). The form _Pelion lyelli_ Wyman was the first known of the Linton Amphibia, and its striking frog-like (123, 639) appearance was early noticed. There is no assurance that the species belongs with this order, but since a well-developed and highly specialized frog (480, 481) occurs in the Como Beds (405) of Wyoming, it is not impossible that we may have in the _Pelion lyelli_ a suggestion (460), at least, of the ancestral structure. It is certain that the frogs have, in past ages, had a much greater length of vertebral column than they possess at present, as is witnessed by the coalescence of several vertebræ to form the urostyle. It is suggested that the ancestral vertebral column is represented in _Pelion_. =Family PELIONTIDÆ Cope, 1875.= Cope, Bull. U. S. Nat. Mus., No. 1, p. II, 1875. The present family includes but a single species, that of _Pelion lyelli_ Wyman, first described in 1858 (640), from Linton, Ohio. The family characters are to be found in the broad and obtusely rounded cranium, in the frog-like scapular arch, the frog-like hind limb, and in the form of the palate, so far as these structures have been preserved. It has been suggested that the present form shows decided affinities with the frogs of to-day and it may possibly be looked upon as the actual ancestor of the living frogs. The length of the vertebral column would seem to militate against such a relationship, since it is well known that frogs have had a short vertebral column since the Jurassic (480, 481). But this is not a good argument, since the developing urostyles of modern tadpoles show metameric fenestrations in the developing bone which doubtless correspond to openings between the vertebræ. The notochord of the tail is segmented, apparently through the influence of former vertebral structure. At any rate, the suggestion is an interesting one and, whether sustained or disproven, the present discussion is based on the probabilities of the case. =Genus PELION Wyman, 1868.= Wyman, Am. Jour. Sci. (2), XXV, p. 160, 1858 (_Raniceps_). Cope, Proc. Acad. Nat. Sci. Phil., 1868, 211 (_Pelion_, suggested in letter to Cope by Wyman). Type: _Pelion lyelli_ Wyman.