The Ferns differ chiefly in some of the details of the leaf. _Pecopteris_, for instance (Fig. 48), have the leaves once, twice, or thrice pinnatifid with the leaflets adhering either by their whole base or by the centre only; the midrib running through to the point. _Neuropteris_ (Fig. 49) has leaves divided like Pecopteris, but the midrib does not reach the apex of the leaflets, but divides right and left into veins. _Odontopteris_ (Fig. 51) has pinnatifid leaves, like the last, but its leaflets adhere by their whole base to the stalk. _Lonchopteris_ (Fig. 50) has the leaves several times pinnatifid, the leaflets more or less united to one another, and the veins reticulated. Among the most numerous species of forms of the Coal-measure period was _Sphenopteris artemisiæfolia_ (Fig. 52), of which a magnified leaf is represented. Sphenopteris has twice or thrice pinnatifid leaves, the leaflets narrow at the base, and the veins generally arranged as if they radiated from the base; the leaflets are frequently wedge-shaped. [Illustration: Fig. 45.--Lepidostrobus variabilis.] [Illustration: Fig. 46.--Lepidodendron elegans.] CARBONIFEROUS LIMESTONE. (SUB-PERIOD.) The seas of this epoch included an immense number of Zoophytes, nearly 400 species of Mollusca, and a few Crustaceans and Fishes. Among the Fishes, _Psammodus_ and _Coccosteus_, whose massive teeth inserted in the palate were suitable for grinding; and the _Holoptychius_ and _Megalichthys_, are the most important. The Mollusca are chiefly Brachiopods of great size. The Productæ attained here exceptional development, _Producta Martini_ (Fig. 53), _P. semi-reticulata_ and _P. gigantea_, being the most remarkable. Spirifers, also, were equally abundant, as _Spirifera trigonalis_ and _S. glabra_. In _Terebratula hastata_ the coloured bands, which adorned the shell of the living animal, have been preserved to us. The _Bellerophon_, whose convoluted shell in some respects resembles the Nautilus of our present seas, but without its chambered shell, were then represented by many species, among others by _Bellerophon costatus_ (Fig. 54), and _B. hiulcus_ (Fig. 56). Again, among the Cephalopods, we find the _Orthoceras_ (Fig. 57), which resembled a straight Nautilus; and Goniatites (_Goniatites evolutus_, Fig. 55), a chambered shell allied to the Ammonite, which appeared in great numbers during the Secondary epoch. [Illustration: Fig. 47.--Lepidodendron Sternbergii.] Crustaceans are rare in the Carboniferous Limestone strata; the genus Phillipsia is the last of the Trilobites, all of which became extinct at the close of this period. As to the Zoophytes, they consist chiefly of Crinoids and Corals. The Crinoids were represented by the genera _Platycrinus_ and _Cyathocrinus_. We also have in these rocks many Polyzoa. [Illustration: Fig. 48.--Pecopteris lonchitica, a little magnified.] [Illustration: Fig. 49.--Neuropteris gigantea.] [Illustration: Fig. 50.--Lonchopteris Bricii.]