The primordial cranium of the Sturgeons consists of persistent cartilage without ossifications in its substance, but superficial bones are still more developed and specialised than in the Dipnoi; so it is, at least, in the true Sturgeons, but less so in _Polyodon_ (Fig. 37). The upper and lateral parts of the skull are covered by well-developed _membrane bones_, which, from this suborder, upwards in the series, will be found to exist throughout the remaining forms of fishes. They are bones, the origin of which is not in cartilage but in membranous connective tissue. The lower surface of the skull is covered by an extremely large basal bone, which extends from the vomerine region on to the anterior part of the spinal column. The nasal excavation in the skull is rather lateral than inferior. The ethmoidal region is generally much produced, forming the base of the long projecting snout. The suspensorium is movably attached to the side of the skull, and consists of two pieces, a hyomandibular and a symplectic, which now appears for the first time as a separate piece, and to which the hyoid is attached. The palato-maxillary apparatus is more complex than in the Sharks and Dipnoi; a palato-pterygoid consists of two mesially-connected rami in _Polyodon_, and of a complex cartilaginous disk in _Acipenser_, being articulated in both to the Meckelian cartilage. In addition, the Sturgeons possess one or two pairs of osseous rods, which, in _Polyodon_ at least, represent the maxillary, and therefore must be the representatives of the labial cartilages of the Sharks. The Meckelian cartilage is more or less covered by tegumentary bones. In the gill-cover, besides the operculum, a sub- and interoperculum may be distinguished in _Acipenser._ The hyoid consists of three pieces, of which the posterior bears a broad branchiostegal in _Polyodon._ In the scapulary arch the primordial cartilaginous elements scarcely differ from those of the _Dipnoi._ The membrane-bones are much expanded, and offer a continuous series suspended from the skull. Their division in the median ventral line is complete. [Illustration: Fig. 38.--Fore-limb of Acipenser.] The pectoral is supported by a cartilaginous framework (Fig. 38) similar to that of _Ceratodus_, but much more shortened and reduced in its periphery, the branches being absent altogether on one side of the axis. This modification of the fin is analogous to the heterocercal condition of the end of the spinous column. To the inner corner of a basal cartilage (_a_) a short axis (_b_) is joined, which on its outer side bears a few branches (_d_) only, the remaining branches (_c_) being fixed to the basal cartilage. The dermal fin-rays are opposed to the extremities of the branches, as in the _Dipnoi_. The _pubic_ consists of a paired cartilage, to which tarsal pieces supporting the fin-rays are attached.