[Illustration]

Other species of Bivalves were also introduced, approaching more nearly our Clams and Oysters, or, as they are called in scientific nomenclature, the Lamellibranchiates.  They differ from the Brachiopods chiefly in the higher character of their breathing-apparatus; for they have free gills, instead of the net-work of vessels on the lining skin which serves as the organ of respiration in the Brachiopods.  We shall always find, that, in proportion as the functions are distinct, and, as it were, individualized by having special organs appropriated to them, animals rise in the scale of structure.  The next class of Mollusks, the Gasteropods, or Univalves, with spiral shells, were numerous, but, from their brittle character, are seldom found in a good state of preservation.

The Chambered Shells, or the Cephalopods, represented chiefly in the earlier periods by the straight Orthoceratites described in a previous article, are now curled in a close coil, and the internal structure of their chambers has become more complicated.  The subjoined wood-cut represents a characteristic Chambered Shell of the Carboniferous age.  Goniatites is the scientific name of these later forms.  If we had looked for them in the Devonian period, we should have found many with looser coils than these, and some only slightly curved in the shape of a horn.  These, as well as the perfectly straight forms, still exist in the coal period, but the Goniatites with close whorls are the more numerous and more characteristic.

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The Articulates have gained their missing class since the close of the Devonian period, for Insects have come in, and that division of the Animal Kingdom is therefore complete, and represented by three classes, as it is at present.  Of the Worms little can be said; their traces are found as before, but they are very imperfectly preserved.  There are still Trilobites, but they are very few in number, and other groups of Crustacea have been added.

One of the most prominent of these new types bears a striking resemblance to the Horse-Shoe Crab of present times.

[Illustration]

[Illustration]

I here present one of our common Horse-Shoe Crabs above one of these old-world Crustaceans, and it will be seen, that, while the latter preserves some of the Trilobitic characters, such as the marked articulations on the posterior part of the body and their division into three lobes, yet in the prominence of its anterior shield, its more elongated form, and tapering extremity, it resembles its modern representative.  In some of them, however, there is no such sharp point as is here figured, and the body terminates bluntly.  There were a large number of these Entomostraca in the Carboniferous period, a group which is chiefly represented among living Crustacea by an exceedingly minute kind of Shrimp; but in those days they were of the size of our Crabs and Lobsters, or even larger, and the Horse-Shoe Crab still maintains their claim to a place among the larger and more conspicuous members of the class.