But there are also some orders that would be best illustrated by a long line of nearly uniform breadth, extending from the first geologic periods to the present day. In other words, they seem to have retained during all time pretty much the same amount of development. I shall confine my notice of the carboniferous Crustacea to the description of a single genus belonging to a family that seems to have begun during the period of the Lower Silurian, and still flourishes abundantly in existing waters. [Illustration: Fig. 20. 1. Carboniferous cypris, nat. size, and magnified. 2. Recent cypris, highly magnified. 3. Carboniferous king-crab (_Limulus trilobitoides_). ] The genus to which I refer is a well-known fossil in some parts of the Coal-measure series, and has been named _Cypris_. The shells of _cyprides_ are very minute, considerably less than the heads of small pins (Fig. 20-1). They can be seen quite well, however, without the use of a magnifying power. In shape they resemble beans, and when seen scattered over a slab of shale, look much liker seeds than the relics of animal life. Yet, under this simple exterior, they concealed a somewhat complex organization. The little bean-shaped shells, which are all that now remains to us of their structure, formed the crust or outer shell in which their viscera were contained, and answered to the massive carapace and segments of the crab. They consisted of two valve-like cases fitting to each other, so as to resemble the united valves of a bivalve shell. From the upper end there were protruded through the opening between the valves a pair of slim jointed antennæ, each furnished at its point with a bundle of minute hair-like cilia (Fig. 20-2). These, when set in rapid motion, served to impel the creature through the water. The legs, four in number, were encrusted with the same hard membrane, and had the same jointed structure as those of our common shrimps and crabs. The foremost pair were pointed like the antennæ with fine hairs, the incessant action of which assisted the animal in swimming. Of the little, confluent, sessile eyes, the delicate branchia or gills, and all the complex internal structure of the nervous, circulating, and other systems, no trace has survived on the stone; but enough of the general external form is left to show us the true affinities of these organisms in the animated world of the present time. By studying the forms and habits of the cyprides that swarm in some of our ponds and marshes, a just conception is obtained of the structure and habitat of the animals that once occupied the minute bean-shaped shells, which lie by millions among the shales of the Carboniferous system. From such a comparison we infer, that just as the cyprides of to-day are fresh-water animals abounding among the green slime of stagnant pools, so, in past ages, they must have preserved with the same organization the same habits. And thus