The specimen described under the above name is a representative of the smallest species of the Microsauria so far described from North America. _Orthocosta microscopica_ Fritsch, from the Carboniferous of Bohemia, is a rival of the present form in size, but the form described by Fritsch is an entirely different animal and was formerly included among the so-called Aistopoda, which are regarded by the writer as merely specialized microsaurians. The present form shows clear affinities with the Microsauria. As may be seen by referring to the list of measurements, the skull of this form measures only 6.5 mm. in length. The form may possibly be larval, though I do not think so, if I may judge from the well-developed condition of the skull bones and the complete ossification of the vertebræ. The sides of the skull are equal and the occiput is a straight table, so that the skull forms almost an exact equilateral triangle. The orbits are very small and are placed well forward. The interorbital space is four times that of the diameter of the orbit, a very unusual character and in itself worthy of ranking as a generic character. The median suture of the skull is zigzag and incloses the minute parietal foramen near the posterior end of the skull. The relations of the elements of the skull, with the exceptions of those of the frontals and parietals, can not be determined with accuracy, although there are here and there indications of sutures. The characters of the cranial elements, so far as they can be determined, are those of the family Tuditanidæ, and the form may, for the present, be regarded as a member of that group. The teeth are very long, slender, and sharp, and are placed close together. There is no indication of fluting on the teeth. They are slightly curved inward. There are 13 vertebræ present. The centra are hour-glass shaped, and are apparently phyllospondylous, with the notochord largely persistent. The vertebral centra are unusually long and slender, with the ends rounded. The humerus of the right side is preserved. It is a long, slender bone with expanded extremities. There is no evidence of abdominal armature nor of ribs (fig. 22, E). The discovery of this form in the Linton deposits is of considerable interest in that it indicates a wide range in size and character of the fauna of the time. The forms now known from the Linton beds range from Odonterpeton, which possibly had a total length of 2 inches in life, to the form designated _Macrerpeton huxleyi_ Cope, with a skull of at least 8 inches in length and whose body may have attained a length of some feet. The large rib described below undoubtedly indicates a large form of the ancient Amphibia from Linton, as do also the vertebræ described by Marsh in 1863 from Nova Scotia. Measurements of the Type.