In order to assist the general reader in following the above descriptions, an outline figure with accompanying explanation is annexed.

5. Probable habits and affinities.- Much is here unavoidably conjectural for the habits and affinities of these extinct forms of life are veiled, to a great extent, in an almost impenetrable obscurity. The following are, perhaps, the only really undebateable points connected with the inquiry. First, the trilobites were marine crustaceans. Their evidently articulated structure and the character of their shelly covering, combined with the compound eyes, which so many of them exhibit, and with their geological conditions of occurrence, are sufficient to establish this. The possession of compound eyes would alone serve to separate them from the oscabrions or chitons, with which they were at one time placed by LATREILLE and other observers. Secondly, the trilobites were gregarious, living in vast communities-as proved by the abundance of their remains in areas of often very limited extent. From this, it has been imagined by the well-known naturalist MacLeay, that they adhered in masses one upon another, after the manner of many of the sedentary mollusks; but the large compound eyes, the ornamented and frequently spine-bearing shells, and the symmetrical habitus, are broadly opposed to this view. Thirdly, feet were either absent, or, if ever present, were of a more or less rudimentary, soft and perishable nature. No traces of these organs, nor of antennæ, have yet been found although from time to time imaginary discoveries of such have been announced.* Fourthly, the trilobites were able, to a cerSee more especially plate 2 in Castelnau's Essai sur le Système Silurien de l'Amérique Septentrionale: 1843.

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tain extent at least, to roll themselves up into a ball. This property amongst crustaceans, is shared by the terrestrial oniscide, and by several marine genera; notably by sphæroma, a genus of small isopodous crustaceans inhabiting the Baltic, the Mediterranean, and other seas. These marine isopods possess, however, peculiar swimming appendages attached to their caudal extremity: a contrivance of which the trilobite was apparently destitute.

in accordance with the views of BURMEISTER, the place usually assigned to the trilobites at the present day, is amongst the phyllopods: or, at least, in the section branchiopoda. With certain probabilities in favor of this distribution, there are yet many considerations against it. A more or less constant motion of the branchial feet would seem to be almost essential to the economy of the branchiopods; but, in the case of the trilobites, a function of this kind can hardly be reconciled with the rolled up condition in which so many of them are found. If, as may be reasonably inferred, this condition were assumed as a protection under the influence of fear, it would probably be retained by the animal for a considerable time. Amongst existing branchiopods, not one appears to have the power of thus contracting itself into a ball; whereas, amongst the isopods, both terrestrial and marine, the property is almost universal. The shell again, in branchiopodous crustaceans, if present at all, is delicate and fragile, and scarcely to be compared in any way with that of the trilobites. Finally, the minute size--and size may be here legitimately considered as a not unimportant element in the inquiry -the minute and often microscopic dimensions of the branchiopods, together with their general conditions of existence, offer further points of dissimilarity. The trilobites were certainly as nearly allied to the isopods as to the branchiopods; and, at the same time, they had certain strong analogies, if not homologies, with the limuli: in the position and aspect of the large compound eyes, for instance ; in many characters of the shell; and to a certain extent in size, and possibly in mode of life. It seems advisable, therefore, to keep them as a distinct order, and so to frame the classification of the crustacea generally, as to shew their relations to the isopods and phyllopods on the one hand, and to the limuli or xiphosura on the other. The chief difficulty is in the collocation of the latter order. To place the limuli with the suctorial parasitic crustacea, according to a still frequently adopted system, is manifestly in opposition to all natural analogies. And, again, if we place them at the end of the class, as a distinct subdivision, their typical relations become altogether lost. Is this, moreover, their proper place? Are

not the limuli far more nearly related than any one of the ordinary entomostracous orders to the decapods? The grand distinction is the well-known character of the mouth-organs. But may we not consider the six pairs of oral feet in the one, to represent an earlier typical condition of the six pairs of foot-jaws in the other? With all their points of difference, at least, a transition from the limuli to the decapods, may certainly be conceived with far less violence to nature, than one between the last-named group and the phyllopods or other entomostraca. On this view, a distribution of the crustacean orders may be arrived at, as shewn in the annexed table. A combined vertical and horizontal reading of the table brings out the affinities of these orders in accordance with the principles discussed above.

We have yet to consider a few other points of inquiry appertaining to this portion of our subject. These are embraced in the following questions:-First, were the trilobites inhabitants of littoral or of deep-sea zones; and secondly, were they of sedentary or of active habits and if the latter, what were their means of locomotion? For the satisfactory determination of these questions, our data are far from complete. Analogy, and the fact of a very general occurrence in ripple-marked shales and other rocks indicative of a littoral origin, would seem to denote a shelving coast-line, or, at least, a moderate depth of water, as the ancient habitat of the trilobite. Trilobites

are found, however, and not unfrequently, in limestone deposits, associated with brachiopods and other forms usually referred to deepsea types. But the brachiopods are now well-known to range from extreme depths up to the very tide-line: and hence their presence in trilobitic rocks, does not speak against the littoral origin of such deposits. In many instances, it is evident, that the paleozoic limestones, as those of other ages, were derived more or less directly from coral reefs; and these reefs may have afforded shelter to many trilobites. Along the inner edge of the great barrier reef of northeastern Australia for example, where in many places a depth of no more than ten or twelve fathoms exists, different species of both brachiopods and crustaceans are often met with.

Whilst some observers imagine the trilobites to have been more or less sedentary, others contend that they must have been in constant motion-swimming, back downwards, at or near the surface of the sea. The truth lies probably between the two. As already pointed out, the presence of eyes is a strong argument against a sedentary existence, and the rolled up condition of body (so commonly witnessed) speaks equally, on the other hand, against a state of constant motion. It is difficult to conceive that these extinct forms could have been endowed with strong swimming powers, for no traces, even under the most favorable circumstances for preservation, of floating appendages have been met with; and their branchial feet, allowing such to have been present, could not have constituted swimming or gans of any force. The unequally balanced extremities of many species, although in part perhaps compensated by the downward extension of the genal angles of the head-shield, is also an obstacle to the satisfactory adoption of this view. At the same time, it should be observed, that their shell, from its general thinness, must have been comparatively light; and the flattened form of body conducive to a certain degree of buoyancy. A slight movement of the flexible thorax and caudal extremity probably formed a sufficient propelling power for the animal's wants. When alarmed, the contraction of the body would enable it to sink with ease into deeper water; and in its power of adhering by its under side to rocks and to the sea-bottom generally, it possessed a further means of defence against its enemies. By this power of adhesion, moreover, individuals may have been carried on floating bodies over a wide range of coast or across open seas, and thus have given rise to colonies in localities far distant from their normal centres. In this manner the extended geographica limits of certain species may perhaps be accounted for.

Little can be suggested with any certainty respecting the food of the trilobite; but by comparison with existing crustaceans, and from the form of the labrum, it may be inferred that these creatures were carnivorous. Soft-bodied radiata, the coral polyp, decaying matter drifted into sheltered bays-such may have formed, in part at least, the sustenance of the trilobite. A further insight into this question might be obtained, could we trace out the compensating agents in Nature's economy, which served to replace the trilobites after these had passed away.

6. Geological relations. The trilobites appear to have been called into existence almost at the earliest dawn of animal life. They die out at the base of the great carboniferous formation, and thus belong entirely to the earlier and middle portions of the paleozoic age. The separate species offer, with few exceptions, admirable test-forms for the various subdivisions of the Silurian and Devonian groups. Even the genera are in many instances restricted to comparatively narrow limits in their upward range. Thus, the genus Trinucleus is unknown above the deposits which mark the limit of the lower Silurians. Asaphus, Illanus, Paradoxides, follow the same law; but other generic forms, Calymene, Phacops, &c., pass upwards, although as a rule with different species, into the higher Silurian and Devonian periods. Phillipsia, very rare in earlier groups, becomes, in the Lower Carboniferous, almost the only remaining type of the class. But these characteristics will be found in full under our enumeration of the more common species belonging to each genus. At present, let us briefly glance at the geological relations of the Crustacea generally.

The decapods comprise three well-marked groups :-the brachyura, anomoura, and macroura. The brachyura, or short-tailed decapods, the highest group, are first met with in the Cretaceous rocks; the anomoura in the Jurassic; and the macroura in the Carboniferous. The entire order is on the increase.

The stomapods are scarcely known in the fossil state. A single species has been met with in the upper tertiaries of Monte Bolca, and a few doubtful forms in the Jurassic and Devonian strata.

The amphipods and the læmodipods are also rare in the fossil condition. The only certain examples are from tertiary beds.

The marine isopods exhibit fossil examples from the Jurassic formations, upwards. Terrestrial species occur only in amber.

The xiphosures—including the pterygotus in this order-date from the upper Silurian. They are on the decline.