The discovery of _Ceratodus_ does not date farther back than the year 1870, and proved to be of the greatest interest, not only on account of the relation of this creature to the other living _Dipnoi_ and _Ganoidei_, but also because it threw fresh light on those singular fossil teeth which are found in strata of Triassic and Jurassic formations in various parts of Europe, India, and America. These teeth, of which there is a great variety with regard to general shape and size, are sometimes two inches long, much longer than broad, depressed, with a flat or slightly undulated, always punctated crown, with one margin convex, and with from three to seven prongs projecting on the opposite margin. [Illustration: Fig. 141.--Dipterus macrolepidotus.] SECOND FAMILY--CTENODODIPTERIDÆ. _Caudal fin heterocercal. Gular plates. Scales cycloid. Two pairs of molars and one pair of vomerine teeth._ Extinct. _Dipterus_ (_Ctenodus_), _Heliodus_ from Devonian strata. THIRD FAMILY--PHANEROPLEURIDÆ. _Caudal fin diphycercal; vertical fin continuous. Gular plates. Scales cycloid. Jaws with a series of minute conical teeth on the margin._ Extinct. _Phaneropleuron_ from Devonian formations, and the carboniferous _Uronemus_ are probably generically identical. FOURTH SUB-ORDER--CHONDROSTEI. _Skeleton notochordal; skull cartilaginous, with dermal ossifications; branchiostegals few in number or absent. Teeth minute or absent. Integuments naked or with bucklers. Caudal fin heterocercal, with fulcra. Nostrils double, in front of the eyes._ FIRST FAMILY--ACIPENSERIDÆ. _Body elongate, sub-cylindrical, with five rows of osseous bucklers. Snout produced, subspatulate or conical, with the mouth at its lower surface, small, transverse, protractile, toothless. Four barbels in a transverse series on the lower side of the snout. Vertical fins with a single series of fulcra in front. Dorsal and anal fins approximate to the caudal. Gill-membranes confluent at the throat and attached to the isthmus. Branchiostegals none. Gills four; two accessory gills. Air-bladder large, simple, communicating with the dorsal wall of the œsophagus._ [Illustration: Fig. 142.--Tail of Acipenser. _a_, Fulcra; _b_, osseous bucklers.]