Page:An Introduction to the Study of Fishes.djvu/103

Rh Polyodon), and the extinct the [sic] Chondrosteidæ, Palæoniscidæ, and (according to Traquair) Platysomidæ.

Their spinal column does not differ essentially from that of the Dipnoi. Segmentation is represented only as far as the neural and hæmal elements are concerned. All are eminently heterocercal. Ribs are present in most, but replaced by ligaments in Polyodon.

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 sub-order, 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