Page:An Introduction to the Study of Fishes.djvu/104

76 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 inter-operculum 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.

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