others, among which the most conspicuous was the Pentacrinite (Fig. 173)-a genus still living in our present seas.

Sea-urchins

FIG. 173. Jurassic Crinoid (Pentacrinus fasciculosus, }).

swarmed on some parts of the sea-floor; among their more frequent genera are Cidaris (Fig. 174), Diadema, Hemicidaris, Acrosalenia, Glyptichus, Pygaster. Of

the contrasts between the Mesozoic and Palæozoic faunas, one of the most marked is to be found among the brachiopods. Except the persistent inarticulate types which have lived on from Cambrian time to the present day (Crania, Lingula, Discina), the

numerous and varied forms which FIG. 174. - Jurassic Sea - urchin played so important a part in the life (Cidaris florigemma, 3), Corallian. of the Palæozoic seas died out almost entirely at the close of the Paleozoic period. The ancient Spirifers and Leptænids

lingered on until the Jurassic period, and then disappeared. On the other hand, the genera Rhynchonella and Terebratula, which occupied a subordinate place in earlier ages, now became the chief representatives of the brachiopods. They abounded throughout Mesozoic time, but they have gradually diminished in number since then, and at the present day each genus survives only in a small number of species. With the decay of the brachiopods, the other divisions of the mollusca

FIG. 175. Jurassic Lamellibranchs. (a) Trigonia monilifera (3), Kimmeridge Clay; (b) Plicatula spinosa (3), Middle Lias; (c) Gryphaa arcuata (incurva) (), Lower Lias.

lamellibranchs

a

proportionately advanced. The attained great development in Mesozoic time, some characteristic genera being Gervillia, Exogyra, Lima, Ostrea, Pecten, Pinna, Astarte, Hippopodium, Trigonia (Fig. 175). Some of the oysters were particularly abundant, Gryphæa, for instance, being so plentiful in some bands of limestone as to give the name of "Gryphite limestone" to them. But undoubtedly the distinctive feature of the molluscan fauna of Mesozoic time was the great development of the cephalopods. The chambered division was represented by an extraordinary variety of Ammonites (Fig. 176), and the cuttle-fishes by many species of Belemnite (Fig. 177). The ammonites have

been made use of to mark off the formations into distinct zones; for, as a rule, the vertical range of each species is comparatively small. The band of strata characterised by a particular species

FIG. 176.-Jurassic Ammonites. (a) Ammonites striatus (), Middle Lias; (b) A. communis (), Upper Lias; (c) A. cordatus (), Lower Calcareous Grit; (d) A. Jason (3), Oxford Clay.

of ammonite is called the zone of that species, e.g. Zone of Amm. planorbis, which is the lowest zone of the Lower Lias.

Another striking contrast is presented by the Jurassic crustacea

when compared with those of the Paleozoic ages. The ancient order of trilobites, so abundant in the seas of the older time, had now entirely disappeared; the eurypterids, which took their place

upon the scene as the trilobites were on the wane, had likewise vanished. In their stead there now came abundant ten-footed crustacea, including both long-tailed forms-the ancestors of our lobsters, prawns, shrimps, and cray-fish-and short-tailed forms that heralded the coming of our living crabs (Fig. 178). Among

the Jurassic strata there occasionally occur thin bands, which have received the name of "insect-beds" from the numerous insect-remains which they contain. The neuroptera are most frequent, but orthoptera and coleoptera also occur. Among these remains are forms of dragon-fly, May-fly, grasshopper, and cockroach. The wing-cases of beetles also are not uncommon; and there has been found the wing of a butterfly-the oldest example of a lepidopterous insect yet known.

Fishes abounded in the waters of the Jurassic time. Those of which the remains have been preserved are chiefly small ganoids (Pholidophorus, Dapedius, Lepidotus, Pycnodus, Fig. 179), with no representatives of the huge bone-cased placoderms of earlier

time. There were likewise various tribes of sharks and rays (Hybodus, Acrodus, Squaloraia).

But, taking the Jurassic fauna as a whole, undoubtedly its most striking character was given by the extraordinary development of its reptiles. So remarkably varied was the reptilian life throughout the Mesozoic period that this part of the earth's history has been called the "Age of Reptiles." There were forms which haunted the sea, others that frequented the rivers, some that lived on the land, some that flew through the air. Never before or since has there been such a profusion of reptilian types. Some of these are still represented at the present time. The Jurassic Teleosaurus and Steneosaurus, for example, have their counterparts now in the living crocodile and alligator. The modern turtles, too, are descendants of those which lived in Jurassic times. But it is the long-extinct types that fill us with astonishment. One of the

FIG. 180.-Jurassic Sea-lizard (Ichthyosaurus communis, g), Lias.

most abundant of them is that of the enaliosaurs or sea-lizards, of which the two leading forms were the Ichthyosaurus and Plesiosaurus. The former creature (Fig. 180), occasionally more than 24 feet long, somewhat resembled a whale in shape and bulk, its head being joined by no distinct neck to the body, which tapered into a long tail. It swam by means of two pairs of strong paddles, and probably steered itself by a fin on the tail. Its eyes were large, and had a ring of bony plates round the eyeball, which remain distinct in the fossil state. Its jaws were armed with numerous strong pointed teeth, not set in distinct sockets. This reptile probably lived chiefly in the sea, feeding there upon the abundant ganoid fishes which its huge protected eyes enabled it to track even into the deeper water. But it no doubt also sought the land, and was able to waddle along the shore or to lie there basking in the sunshine. The Plesiosaurus, in many respects like the Ichthyosaurus, was distinguished by its proportionately shorter tail, longer neck, smaller head, larger paddles, and the insertion of the teeth in distinct sockets. It probably haunted the lagoons, rivers, and shallow seas of the time.

Its