They are likely to be found anywhere in the limestone belt, from the Appalachian Mountains to the Rocky Mountains, or in the Great Basin, or on the Pacific Coast. Often they have been mistaken for turtles and other objects. A good many of the cases where the head or body of animals “petrified with all the flesh” are reported, it is one of these concretions which has a shape sufficiently like the part described, for the imagination to construct the rest. Septeria Pl. 69 Septeria are lime concretions, which, after they had formed, have shrunk and developed a series of cracks running through them in all sorts of directions, and since then the cracks have been filled with various minerals, such as calcite, dolomite, and siderite. These make a series of veins which intersect the concretion, in a sort of network. Septeria are mostly of considerable size, ranging from six inches in diameter to several feet through. They are characteristic of the shales of ancient sea bottoms, especially those of Devonian age in New York, and Pennsylvania, and those of Cretaceous age in Wyoming, Montana and the Dakotas. Flint concretions The silica in limestones is often segregated into nodular masses of varying sizes, to make concretions of flint. Such masses have grown in the limestone, and, while growing, have either pushed away, or dissolved the adjacent limestone, so that the flint nodule is pure silica. They are especially characteristic of the chalk beds, and of ancient limestones which formed on the floor of the sea, like the Helderberg Limestone of New York, Pennsylvania, Ohio, etc. When thin sections are cut through these flints, and examined under the microscope, many remnants of the shells of plants and animals are still recognizable. A nucleus is seldom found, but in some cases there is a fossil in the nodule about which the concretion doubtless formed. The spicules of sponges, shells of diatoms, and of radiolarians seem to have contributed most of the material from which flint concretions are formed. In addition to the silica there are frequently inclosed in these nodules the horny jaws of various sea worms, and a host of spiny balls the relationships of which are still unknown. Sandstone concretions There are two types of sandstone concretions, first those which are cemented with lime, and second those cemented with iron oxide. The concretions bound by lime are especially characteristic of sandstones which were laid down as river deposits, either in the channels or on the flood plains, and also the sandy deposits resulting from wind deposition. In these cases the concretions will mostly be found to have formed around some organic nucleus, most frequently about a bone, or group of bones, of some ancient animal. In this country they are mostly found in the arid and semiarid sections of the West, where the present day wind erosion exposes the harder parts of bluffs, etc.