Does the reader possess a copy of Lyell's lately published elementary work, edition 1838? If so, let him first turn up the description of the Upper Silurian rocks, from Murchison, which occurs in page 459, and mark the form of the trilobite _Asaphus caudatus_, a fossil of the Wenlock formation. (See _Sil. Sys._, Plate VII.) The upper part, or head, forms a crescent; the body rises out of the concave with a sweep somewhat resembling that of a Gothic arch; the outline of the whole approximates to that of an egg, the smaller end terminating in a sharp point. Let him remark, further, that this creature was a _crustaceous_ animal, of the crab or lobster class, and then turn up the brief description of the Old Red Sandstone in the same volume, page 454, and mark the form of the _Cephalaspis_, or buckler-head--a _fish_ of a formation immediately over that in which the remains of the trilobite most abound. He will find that the fish and the crustacean are wonderfully alike. The fish is more elongated, but both possess the crescent-shaped head, and both the angular and apparently jointed body.[L] They illustrate admirably how two distinct orders may meet. They exhibit the points, if I may so speak, at which the plated fish is linked to the shelled crustacean. Now, the _Coccosteus_ is a stage further on; it is more unequivocally a fish. It is a _Cephalaspis_ with an articulated tail attached to the angular body, and the horns of the crescent-shaped head cut off. [Footnote L: Really jointed in the case of the trilobite; only apparently so in that of the _Cephalaspis_. The body of the trilobite, like that of the lobster, was barred by transverse, oblong, overlapping plates, and between every two plates there was a joint; the body of the _Cephalaspis_, in like manner, was barred by transverse, oblong, overlapping scales, between which there existed no such joints. It is interesting to observe how nature, in thus bringing two such different classes as fishes and crustacea together, gives to the higher animal a sort of pictorial resemblance to the lower, in parts where the construction could not be identical without interfering with the grand distinctions of the classes.] Some of the specimens which exhibit this creature are exceedingly curious. In one, a coprolite still rests in the abdomen; and a common botanist's microscope shows it thickly speckled over with minute scales, the indigestible exuviæ of fish on which the animal had preyed. In the abdomen of another we find a few minute pebbles--just as pebbles are occasionally found in the stomach of the cod--which had been swallowed by the creature attached to its food. Is there nothing wonderful in the fact, that men should be learning at this time of day how the fishes of the Old Red Sandstone lived, and that there were some of them rapacious enough not to be over nice in their eating?