During the Ordovician Period, 425 to 500 m.y. ago, upper sands of the Bliss as well as overlying limestones and dolomites were deposited in the shallow warm seas of southern New Mexico; these latter rocks are the El Paso Limestone and Montoya Dolomite. The Ordovician seas teemed with invertebrate life. Fifteen-foot-long cephalopods, as much as a foot in diameter, ruled the shallow salt-water bottoms, munching on the abundant trilobites and the moss animals, the bryozoans. Numerous brachiopods, corals, snails, and clams also thrived, with many of the Ordovician carbonate-rock beds literally being made up of these fossil remains. Near El Paso, these limy fossiliferous beds are nearly 2000 feet thick, but they thin northward to a knife edge in thickness near Mockingbird Gap at the north end of the San Andres Mountains. Parts of northwestern New Mexico may have been low islands exposed to the sun and erosion during Ordovician time, but most of the state was probably within an extensive shallow ocean. Later, erosion removed the Ordovician rocks from central and northern New Mexico. Silurian strata, the brown Fusselman Dolomite, deposited during the middle of that period (400 to 425 m.y. ago), remain only in the southern and southeastern parts of the state, thinning out northward from the 1000-foot-thick bed near El Paso. The extent of these middle Silurian seas is not known, but most of central and northern New Mexico was undergoing erosion during late Silurian time. The northward thinning of the Fusselman Dolomite is due chiefly to this erosion, evidenced by the knobby, ridged and channeled top surface of the Fusselman. Brachiopods and corals are the most abundant fossils in the Fusselman Dolomite; elsewhere, Silurian rocks are known for the sea scorpions or eurypterids, which attained a length of nine feet, and for the complete remains of primitive fishes. During early and middle Devonian time (345 to 400 m.y. ago), most of New Mexico was a lowland rotting beneath the sun. Fossiliferous Devonian rocks are unknown in the north-central part of the state but occur beneath the surface in the Four Corners region of northwestern New Mexico and the adjoining states. These rocks are of late Devonian age and consist of lower dolomite and sandstone, middle shale and dolomite, and the upper Ouray Limestone. In southern New Mexico, a uniform blanket of dark limy muds, called the Percha Shale, was deposited during late Devonian time. This shale marks a great change from the limestones of earlier ages. In part, it is of black muds deposited in widespread or in local stagnant basins and in part calcareous fossiliferous muds in which abundant invertebrate life was buried. The clay and quartz silt that make up the rocks were a weathered residuum that had accumulated, during the long period of late Silurian and early and middle Devonian times, on the lowland of central and northern New Mexico.