_An_, Angular; _ao_, ante-orbital; _Ar_, articulary; _B_, basal; _D_, dentary; _E_, ethmoid; _F_, frontal; _Ma_, mastoid; _Mp_, metapterygoid; _Mx_, maxillary; _N_, nasal; _O_, operculum; _Oc_, occipital; _Pa_, parietal; _Pl_, palatine; _Pmx_, præmaxillary; _po_, post-orbital; _Prf_, prefrontal; _Pt_, post-temporals; _Ptf_, postfrontal; _Ptr_, pterygoid; _Q_, quadrate; _S_, suspensorium; _So_, sub-operculum; _Sp_, sphenoid; _Spl_, splenial; _St_, supratemporals; _T_, tympanic lamina; _Tu_, turbinal; _v_, vomer; _x x_, small ossicles; _x’ x’_, spiraculars.] Only a very small portion of the bones described are visible externally, nearly the whole of the primordial cranium being covered by the membrane-bones. Of these are seen on the upper surface a pair of parietals, frontals, “nasals,” and turbinals; on the lower surface a large cross-shaped basal, anteriorly bordered on each side by a pterygoid, parallel to a palatine which forms a suture with the double vomer. The suspensorium has in front a metapterygoid and quadrate bone, and an operculum and sub-operculum are attached to it behind. Præmaxillaries and maxillaries are now fully developed, but immovably attached to the skull. The lower jaw is ossified, and consists of an articulary, angular, dentary, and splenial. Of labial cartilages a rudiment at the angle of the mouth has remained persistent. The side of the skull, in front of the operculum, is covered by a large irregularly-shaped bone (_T_) (corresponding to the “tympanic lamina” of _Ceratodus_, Fig. 35, _q_), held by some to be the præoperculum; along its upper circumference lies a series of small ossicles, of which two may be distinguished as spiraculars, as they form a valve for the protection of the spiracular orifice of these fishes. An infraorbital ring is represented by a præ- and post-orbital only. Each _hyoid_ consists of three pieces, none of which bear branchiostegals, the single median piece being osseous in front and cartilaginous behind. Four branchial arches are developed, the foremost consisting of three, the second and third of two, and the last of a single piece. There is no lower pharyngeal. Between the rami of the lower jaw the throat is protected by a pair of large osseous laminæ (_gular plates_), which have been considered to represent the urohyal of osseous fishes. The scapulary arch is almost entirely formed by the well-developed membrane-bones, which in the ventral line are suturally united. The pectoral fin is supported by three bones, pro-, meso-, and metapterygium, of which the dilated middle one alone bears rays, and is excluded from the articulation with the shoulder-girdle. The pubic consists of paired bone, to which tarsal bones supporting the fin-rays are attached.