(_l_) The ironstone nodules, in which the Mazon Creek fossils (plate 1) occur, are found in the shale which forms the roof of the Morris or "No. 2" Coal of Illinois, which "lies probably somewhat lower than the horizon of the Lower Kittanning Coal of Pennsylvania" (599). "The nodules of iron contained in the Coal shales on the banks of Mazon Creek near Morris, Illinois, generally contain organic nuclei, and thousands of beautiful specimens have been obtained there. They are usually fragments of fern fronds, but are sometimes shells, crustaceans, myriapods, scorpions, spiders, cockroaches, ... fishes" (498, p. 214), and amphibians, of which 10 species are at present known. [Illustration: MOODIE PLATE 1 VIEWS ALONG MAZON CREEK, ILLINOIS. 1. A nodule weathering out of the shale, at the head of the hammer. Most of the nodules at the so-called "lower beds" contain specimens of _Neuropteris_. 2. The nodules in the creek bed at the "upper beds." Many of them have been cracked open by the frost. 3. Looking south at the "upper beds." The nodules found in the background are non-fossiliferous. 4. Nodules may be seen through the clear water embedded in the shale. Neuropterid insects in the water. 5. Looking for nodules at the "upper beds." The uppermost reaches of the fossiliferous beds correspond with the extreme background of the picture. 6. Nodules in the stream bed at the "lower beds." Many of these are cracked open by the frost and good specimens are sometimes found in the nodules. ] These species have been arranged zoologically according to the following plan: Class Amphibia Linné, 1758. Subclass Euamphibia Moodie, 1909. Order Branchiosauria Lydekker, 1889. Family _Branchiosauridæ_ Fritsch, 1879. _Micrerpeton caudatum_ Moodie, 1909. _Eumicrerpeton parvum_ Moodie, 1910. _Mazonerpeton longicaudatum_ Moodie, 1912. _Mazonerpeton costatum_ Moodie, 1912. Order Caudata Duméril, 1806. Suborder Proteida Cope. Family _Cocytinidæ_ Moodie, 1912. _Erierpeton branchialis_ Moodie, 1912. Subclass Lepospondylia Zittel, 1887. Order Microsauria Dawson, 1863. Family _Amphibamidæ_ Cope, 1875. _Amphibamus grandiceps_ Cope, 1865. _Amphibamus thoracatus_ Moodie, 1911. _Cephalerpeton ventriarmatum_ Moodie, 1912. Family _Molgophidæ_ Cope, 1875. _Erpetobrachium mazonensis_ Moodie, 1912. Subclass Stegocephala Cope, 1868. Order Temnospondylia Zittel, 1887. Suborder Embolomeri Cope, 1885. Family _Cricotidæ_ Cope, 1884. _Spondylerpeton spinatum_ Moodie, 1912. It will be seen from the above arrangement that nearly all of the orders of Amphibia are represented in the Mazon Creek fauna. These animals are the oldest known land vertebrates of North America.