The _Nilssonia_, finally, were Cycadeaceæ resembling the Pterophyllum, but with thick and coriaceous leaves, and short leaflets contiguous to, and in part attached to the base; they were obtuse or nearly truncated at the summit, and would present nervures arched or confluent towards that summit. The essential characters of the vegetation during the Liassic sub-period were:--1. The great predominance of the Cycadeaceæ, thus continuing the development which commenced in the previous period, expanding into numerous genera belonging both to this family and that of the _Zamites_ and _Nilssonia_; 2. The existence among the Ferns of many genera with reticulated veins or nervures, and under forms of little variation, which scarcely show themselves in the more ancient formations. [Illustration: Fig. 108.--Millepora alcicornis. (Recent Coral.)] [Illustration: XVI.--Ideal Landscape of the Liassic Period.] On the opposite page (PLATE XVI.) is an ideal landscape of the Liassic period; the trees and shrubs characteristic of the age are the elegant Pterophyllum, which appears in the extreme left of the picture, and the Zamites, which are recognisable by their thick and low trunk and fan-like tuft of foliage. The large horsetail, or Equisetum of this epoch, mingles with the great Tree-ferns and the Cypress, a Conifer allied to those of our own age. Among animals, we see the Pterodactyle specially represented. One of these reptiles is seen in a state of repose, resting on its hind feet. The other is represented, not flying, after the manner of a bird, but throwing itself from a rock in order to seize upon a winged insect, the dragon-fly (_Libellula_), the remains of which have been discovered, associated with the bones of the Pterodactyle, in the lithographic limestone of Pappenheim and Solenhofen. OOLITIC SUB-PERIOD. This period is so named because many of the limestones entering into the composition of the formations it comprises, consist almost entirely of an aggregation of rounded concretionary grains resembling, in outward appearance, the roe or eggs of fishes, and each of which contains a nucleus of sand, around which concentric layers of calcareous matter have accumulated; whence the name, from ωον, _egg_, and λιθος, _stone_. The Oolite series is usually subdivided into three sections, the _Lower_, _Middle_, and _Upper Oolite_. These rocks form in England a band some thirty miles broad, ranging across the country from Yorkshire, in the north-east, to Dorset, in the south-west, but with a great diversity of mineral character, which has led to a further subdivision of the series, founded on the existence of particular strata in the central and south-western counties:-- UPPER. MIDDLE. LOWER.