As regards the general distribution of the Crustacea in time, remains of the class are comparatively abundant in all formations except the very oldest; as might have been expected from the generally chitinous or sub-calcareous nature of their integuments and their aquatic habits. Owing also to their habit of periodically casting their shell, a single individual may leave repeated traces of himself, and the number of fossils may considerably exceed that of the individuals which actually underwent fossili-sation. The Crustaceans appear to have commenced their existence in the Cambrian period, remains of members of this class being tolerably abundant in the higher portion of this formation. The Palaeozoic formations, taken as a whole, are characterised by the predominance of the orders Trilobita, Eurypterida, Ostracoda, and Phyllopoda, of which the two former are exclusively confined to this period. All the other orders of Crustacea which have left any traces of their past existence at all, appear to have come into existence before the close of the Palaeozoic period. Upon the whole, however, there has been a marked progression in proceeding from the older formations to the present day. The Trilobites and Eurypterids of the older Palaeozoic rocks, though highly organised so far as their type is concerned, are in many respects inferior to later forms, whilst they present some striking points of resemblance to the larval forms of the higher groups. The great group of the Stalk-eyed Crustaceans - undoubtedly the highest of the entire class - is not represented at all till we reach the Devonian rocks; and it is not till we come into the Secondary period that we find any great development of this group, whilst its abundance increases to a marked extent in the Tertiary period, and it attains its maximum at the present day. Similarly, of the two orders of the Merostomata, the Eurypterida are confined to the earlier portion of the Palaeozoic period, whilst the more highly organised and less larval King-crabs (Xiphosura) hardly made their appearance till the Eurypterids had disappeared, at the close of the Carboniferous period.
Cirripedia. The Cirripedes are hardly known as Palaeozoic fossils, but valves of a singular member of this order (Turrilepas) have been found in the Silurian rocks. With few exceptions, the Cirripedes are entirely confined in past time to the Secondary and Tertiary epochs. The Bala-nidae are the most common, commencing, with the doubtful exception of a Liassic form, in the Chalk, and attaining their maximum in recent seas. The Verrucidae commence in the Chalk, and the Lepadidae, with one or two exceptions, begin in the Jurassic rocks, and attain their maximum of development in the Cretaceous epoch. The Upper Silurian genus Turrilepas, above mentioned, is also referable to the Lepadoids.
Ostracoda. Small Ostracode Crustacea are extremely abundant as fossils in many formations, and extend from the Cambrian period up to the present day.
Phyllopoda. Remains of Crustaceans supposed to belong to this order are found in the Palaeozoic rocks. Hymenocaris is found in the Upper Cambrian, Caryocaris in the Lower Silurian, Ceratiocaris in the Upper Silurian, and Dithyrocaris in the Carboniferous Limestone. All these forms, with other similar ones, are believed to be most closely allied to the recent Apus and Nebalia. The genus Estheria, represented by many forms from the Devonian period to the present day, is also to be referred here.
Trilobita. The Trilobites are exclusively Palaeozoic fossils. In the Upper Cambrian rocks - the so-called "primordial zone" - there occurs a singular group of Trilobites - the so-called primordial Trilobites - distinguished by the possession of many larval characters. In the Lower and Upper Silurian rocks the Trilobites attain their maximum of development. They are still well represented in the Devonian rocks ; but they die out completely before the close of the Carboniferous epoch, being represented in the Mountain Limestone by four genera only (Phillipsia, Brachymetopus, Proetus, and Griffithides).
Eurypterida. These, like the last, are entirely Palaeozoic, attaining their maximum in the Upper Silurian and Devonian formations, and dying out in the Carboniferous rocks. Pterygotus, Eurypterus, and Slimonia are the most characteristic genera.
Xiphosura. The genus Limulus commenced, as far as is yet known, in the Permian period, and. has survived up to the present day. Its first appearance, therefore, was just at the close of the Palaeozoic epoch. Of the remaining genera, which constitute with Limulus this sub-order, Belinurus, Euproops, and Prestwichia, are Palaeozoic, and are not known to occur out of the Carboniferous rocks. The genus Neolimulus is Upper Silurian.
Amphipoda. The oldest known Amphipod is the Necrogammarus of the Upper Silurian.
Isopoda. The earliest known Isopod is the Praearcturus of the Devonian rocks.
Decapoda. The Macrurous Decapods commence their existence in the Carboniferous period, or perhaps in the Devonian, with a few Prawnlike forms ; and the Brachyura seem to have existed at the same period. The Decapoda are, however, well represented, in all their three tribes, in the Secondary and Tertiary epochs, attaining their maximum at the present day. The London Clay (Eocene) is especially rich in the remains of Macrura and Brachyura.