Page:An Introduction to the Study of Fishes.djvu/94

66 There is no trace of ribs or limbs.


 * [J. Müller, Vergleichende Anatomie der Myxinoiden. Erster Theil. Osteologie und Myologie, in Abhandl. Ak. Wiss. Berlin, 1835.]

The Chondropterygians exhibit a most extraordinary diversity in the development of their vertebral column; almost every degree of ossification, from a notochord without a trace of annular structure to a series of completely ossified vertebræ being found in this order. Sharks, in which the notochord is persistent, are the Holocephali (if they be reckoned to this order, and the genera Notidanus and Echinorhinus). Among the first, Chimæra monstrosa begins to show traces of segmentation; but they are limited to the outer sheath of the notochord, in which slender subossified rings appear. In Notidanus membranous septa, with a central vacuity, cross the substance of the gelatinous notochord. In the other Sharks the segmentation is complete, each vertebra having a deep conical excavation in front and behind, with a central



canal through which the notochord is continued; but the degree in which the primitive cartilage is replaced by