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Rh, and Lower Permian, but best in Cricotus (Fig. 76 ) from Illinois and Texas. Rhachitomous vertebrae are much more widely known in numerous forms from the Pennsylvanian and Permian of various parts of the world.

An embolomerous vertebra is composed of two subequal, notochordal disks, the anterior one the intercentrum, or hypocentrum, bearing the exogenous chevron in the tail; the posterior one the pleurocentrum; and the arch or neurocentrum, resting upon both the intercentrum and pleurocentrum, but chiefly the latter. The articular surface for the head or capitulum of the ribs is chiefly on the intercentrum; the surface for the articulation of the tubercle of the rib, on either the arch or diapophysis.

A rhachitomous vertebra (Fig. 76 ) differs in that the intercentrum or hypocentrum is more or less wedge-shaped, with its base on the ventral line, its apex not reaching the dorsal side; while the pleurocentra behind are paired, with the basal side above and their apices reaching the ventral line only narrowly or not at all. The neurocentrum, as in the embolomerous forms, is borne by all three bones, but chiefly by the pleurocentra. The head of the ribs articulates with the intercentrum, the tubercle with the diapophysis of the neurocentrum.

The earliest known amphibian vertebrae are embolomerous; rhachitomous and holospondylous vertebrae appearing later, so far as our present knowledge goes. And this is one of the reasons why it would seem that the embolomerous type is the more primitive, giving origin directly to the reptilian holospondylous type, as was first suggested by Cope; that the rhachitomous type was derived from it by the loss of the upper part of the intercentrum and the lower part of the pleurocentrum and the division of the latter into two lateral parts. This reversion of the pleurocentrum to a more primitive ontogenetic condition is the chief objection to this theory, nevertheless it is the more probable. We have seen that the more primitive phylogenetic condition of the intercentra persists longest in the neck and tail. In the caudal vertebrae of Eryops (Fig. 76 ), and probably other rhachitomous amphibians, there is an intermediate condition between the embolomerous and rhachitomous types, in which the single pleurocentrum is typically embolomerous, that is, disk-like and perforated for the notochord; while the intercentrum bearing