A very primitive type of vertebral column occurs in some of the Jurassic allies of _Amia_, in which certain of the vertebral components, confluent in the adult _Amia_, retain some measure of their primitive distinctness.[185] Thus, in the precaudal region of _Eurycormus_ (Fig. 117, B) there is a series of alternating dorsal and ventral half-rings of bone, which between them completely invest the persistent notochord. Each ventral half-ring or "hypocentrum" represents a pair of fused and ossified basi-ventrals, and possibly also a pair of included inter-ventrals, and supports dorsally a pair of basi-dorsals, forming a neural arch, and laterally a pair of ribs. The dorsal semi-rings, or "pleuro-centra," similarly represent fused and ossified pairs of inter-dorsals. In the tail, modifications approximating to what is seen in the caudal region of _Amia_ are to be noticed (C). By the upgrowth of the ventral arch-bearing semi-rings, and their conversion into complete rings encircling the notochord, incipient pre-centra are formed, and by a similar modification of the {204}down-growing, archless, dorsal half-rings, structures comparable to post-centra are produced. In brief, _Eurycormus_, as well as such other extinct Amioid genera as _Caturus_ (Fig. 117, A), _Callopterus_, and _Euthynotus_, retain in the adult a stage of vertebral evolution which is closely paralleled by transitory stages in the embryonic and young forms of _Amia_. [Illustration: FIG. 118.—A, two vertebrae from the trunk-region of _Lepidosteus_; B, anterior face of a vertebra. _c.n_, Anterior convex face of the centrum; _c.n′_, posterior concave face; _h.a_, parapophysis, with its articular facet for a rib; _i.c_, median cartilage, representing a pair of fused supra-interdorsals; _i.s_, radial element of the dorsal fin; _l.l_, superior longitudinal ligament; _n.a_, neural arch. (From Wiedersheim, after Balfour and W. N. Parker.)] _Lepidosteus_[186] is unique amongst existing Fishes in having opisthocoelous vertebrae; that is, the centra are convex in front and concave behind, and therefore articulate with one another by ball-and-socket joints (Fig. 118). This condition is due to the development of a series of intervertebral rings of cartilage round the notochord. The subsequent inward growth of each of these rings leads to the constriction, and ultimately to the complete obliteration, of the notochord, much in the same way as by the growth of ordinary centra. Later, this solid mass of cartilage becomes transversely divided by a cleft which is convex anteriorly and concave behind (Fig. 116, C), and of the two portions one fuses and co-ossifies with the centrum of the vertebra in front, and the other with the one pertaining to the vertebra behind. Reference to Fig. 116 will show that the grouping of the vertebral elements to form the individual vertebrae is not the same as in _Amia_.