The genus _Micrerpeton_, of which the single species is described below, was the first evidence of the occurrence of the Branchiosauria in America. There have been three other genera referred to the Branchiosauria from North American deposits, but there is good evidence that none of them belong there. The genus Amphibamus was originally referred to the _Xenorhachia_ by Cope (105, pp. 134-137) on account of the supposed cartilaginous condition of the vertebræ and the absence of ribs. Later he abandoned this order and placed the form in the Branchiosauria, where it was retained by Zittel (642). Recently Hay has shown (316), and I am able to corroborate his statement, that ribs are present in the species, and that they are long and curved, not at all like the short ribs of the true Branchiosauria. These long, curved ribs undoubtedly exclude _Amphibamus_ from the Branchiosauria and indicate its close affinities with the Microsauria. The genus _Pelion_ has also been referred to this order on purely negative evidence (642, p. 375). This genus is excluded from the Branchiosauria by the well-ossified condition of the limb bones, in which the endochondral ossification is seen to be well developed, a condition not found, so far, among the true Branchiosauria. The form of the head and the elongate hind limb would also tend to exclude the genus from the Branchiosauria. In the Branchiosauria the fore limb is usually larger than the hind limb, the reverse of which is the case in _Pelion_. The genus _Sparodus_, as it occurs in North America, is uncertain. It is indicated by remains which are almost impossible of determination.