[Illustration: FIG. 116.—A small Carboniferous Ganoid (_Palæoniscus (Rhadinichthys) Modulus_ Dn.). Lower Carboniferous, New Brunswick. _a_, Outline. _b_, _c_, _d_, Sculpture of scales magnified.] [Illustration: FIG. 117.—Teeth and Spines of Carboniferous Sharks. Nova Scotia. _a_, _Diplodus penetrans_. _b_, _Psammodus_. _c_, _Ctenoptychius cristatus_. _d_, Spine, _Gyracanthus magnificus_. One-eighth natural size.—_Acadian Geology._] The Sharks in the Carboniferous increase in number and importance. Fig. 117 shows a few examples of their teeth and spines. In the Carboniferous, however, there is a great preponderance of those species with flat, crushing teeth fitted for grinding shells,[40] which in diminishing numbers continue up to the present time, when they are represented by the Port Jackson Shark and a few other species. The increase toward the modern time of the true Sharks[41] with sharp cutting teeth, is obviously related to the increase of the ordinary fishes which furnish them with food. Another curious difference, connected probably with the same circumstance, is the fact that in the sharp toothed Sharks of the Carboniferous the two side fangs of each tooth are the largest, or are exclusively developed (Fig. 117, _a_), while in later periods the central point becomes dominant, or is developed to the exclusion of the others (Figs. 118, 119). [Illustration: FIG. 118.—Teeth of Cretaceous Sharks (_Otodus_ and _Ptychodus_).—After Leidy.] [Illustration: FIG. 119.—Tooth of a Tertiary Shark (_Carcharodon_).] [Illustration: FIG. 120.—A Liassic Ganoid (_Dapedius_). Restored.—After Nicholson.]