=Order Pycnodonti.=—In the extinct order _Pycnodonti_, as recognized by Dr. O. P. Hay, the notochord is persistent and without ossification, the body is very deep, the teeth are always blunt, the opercular apparatus is reduced, the dorsal fin many-rayed, and the fins without fulcra. The scales are rhombic, but are sometimes wanting, at least on the tail. Many genera and species of _Pycnodontidæ_ are described, mostly from Triassic and Jurassic rocks of Europe. Leading European genera are _Pycnodus_, _Typodus_ (_Mesodon_), _Gyrodus_, and _Palæobalistum_. The numerous American species belong to _Typodus_, _Cœlodus_, _Pycnodus_, _Hadrodus_, and _Uranoplosus_. These forms have no affinity with _Balistes_, although there is some resemblance in appearance, which has suggested the name of _Palæobalistum_. [Illustration: FIG. 12.—_Gyrodus hexagonus_ Agassiz. Family _Pycnodontidæ_. Lithographic Shales. ] Woodward places these fishes with the _Semionotidæ_ and _Halecomorphi_ in his suborder of _Protospondyli_. It seems preferable, however, to consider them as forming a distinct order. [Illustration: FIG. 13.—_Mesturus verrucosus_ Wagner. Family _Pycnodontidæ_. (After Woodward.) ] =Order Lepidostei.=—We may place, following Eastman's edition of Zittel, the allies and predecessors of the garpike in a single order, for which Huxley's name _Lepidostei_ may well be used. In this group the notochord is persistent, and the vertebræ are in various degrees of ossification and of different forms. The opercles are usually complete, the branchiostegals present, and there is often a gular plate. There is no infraclavicle and the jaws have sharp teeth. The fins have fulcra, and the supports of the fins agree in number with the rays. The tail is more or less heterocercal. The scales are rhombic, arranged in oblique series, which are often united above and below with peg-and-socket articulations. This group contains among recent fishes only the garpikes (_Lepisosteus_). They are closely allied to the _Palæoniscidæ_, but the skeleton is more highly ossified. On the other hand they approach very closely to the ancestors of the bowfin, _Amia_. One genus, _Acentrophorus_, appears in the Permian; the others are scattered through Mesozoic and Tertiary rocks, the isolated group of gars still persisting. In the gars the vertebræ are concavo-convex, with ball-and-socket joints. In the others the vertebræ are incomplete or else double-concave, as in fishes generally. For the group here called _Lepidostei_ numerous other names have been used corresponding wholly or in part. _Rhomboganoidea_ of Gill covers nearly the same groups; _Holostei_ of Müller and _Hyoganoidea_ of Gill include the _Halecomorphi_ also; _Ginglymodi_ of Cope includes the garpikes only, while _Ætheospondyli_ of Woodward includes the _Aspidorhynchidæ_ and the garpikes. [Illustration: FIG. 14.—_Semionotus kapffi_ Fraas, restored. Family _Semionotidæ_. (After Fraas, per Nicholson.) ]