=Teleosts.= The great group of fishes called the teleosts, or those with complete bony skeletons, to which most modern fishes belong, may be mentioned here, although in the Devonian they had not yet appeared. The teleosts are a highly specialized type, adapted most perfectly to their aquatic environment. Heavy armor has been discarded, and reliance is placed instead on swiftness. The skeleton is completely ossified and the notochord removed. The vertebræ have been economically withdrawn from the tail, and the cartilaginous axis of the side fins has been found unnecessary. The air sac has become a swim bladder. In this complete specialization they have long since lost the possibility of evolving into higher types. It is interesting to note that the modern teleosts in their embryological growth pass through the stages which characterized the maturity of their Devonian ancestors; their skeleton is cartilaginous and their tail fin vertebrated. CHAPTER XIX THE CARBONIFEROUS The Carboniferous system is so named from the large amount of coal which it contains. Other systems, from the Devonian on, are coal bearing also, but none so richly and to so wide an extent. Never before or since have the peculiar conditions been so favorable for the formation of extensive coal deposits. With few exceptions the Carboniferous strata rest on those of the Devonian without any marked unconformity; the one period passed quietly into the other, with no great physical disturbances. The Carboniferous includes three distinct series. The lower is called the _Mississippian_, from the outcrop of its formations along the Mississippi River in central and southern Illinois and the adjacent portions of Iowa and Missouri. The middle series is called the _Pennsylvanian_ (or Coal Measures), from its wide occurrence over Pennsylvania. The upper series is named the _Permian_, from the province of Perm in Russia. =The Mississippian series.= In the interior the Mississippian is composed chiefly of limestones, with some shales, which tell of a clear, warm, epicontinental sea swarming with crinoids, corals, and shells, and occasionally clouded with silt from the land. In the eastern region, New York had been added by uplift to the Appalachian land which now was united to the northern area. From eastern Pennsylvania southward there were laid in a subsiding trough, first, thick sandstones (the Pocono sandstone), and later still heavier shales,--the two together reaching the thickness of four thousand feet and more. We infer a renewed uplift of Appalachia similar to that of the later epochs of the Devonian, but as much less in amount as the volume of sediments is smaller. The Pennsylvanian Series