vill VERTEBRAL COLUMN 205 
In the dominant group of existing Fishes, the Teleostei, the 
centra are almost invariably biconcave, although in the Eels they 
may be flat or even slightly convex in front. Ribs are absent in 
the Syngnathidae and in the Plectognathi. In addition to the 
usual articulation between the centra, the vertebrae often articu- 
late with one another by means of paired processes arising from 
the anterior margin of each neural arch, or from the centrum 
at the base of the arch (pre-zygapophyses), and meeting similar 
processes which project either from the hinder margin of the arch 
of the vertebra in front, or from the adjacent portion of its centrum 
(post-zygapophyses). The haemal arches may have similar pro- 
Fic. 119.—A, side view of precaudal vertebrae of a Cod (Gadus morrhua) without the 
ribs ; B, similar view of caudal vertebrae of the same Teleost. c, Centrum; h.a, 
haemal arch ; h.sp, haemal spine; 7.a, neural arch; n.sp, neural spine; p, para- 
pophysis ; p.2, pre-zygapophysis ; pt.z, post-zygapophysis. 
cesses (Fig. 119). One, two, or in some Teleosts, three pairs of 
slender intermuscular bones radiate outwards from the centra 
into the myocommata (epicentrals), or from the neural arch 
(epineurals), or from the ribs (epipleurals). 
The Ribs.—lIt is doubtful if the structures termed “ribs” 
are homologous in the different groups of Fishes. There appear 
to be two kinds, distinguishable as dorsal and ventral ribs (Fig. 
156). Dorsal ribs are situated in the fibrous tissue separating 
the epiaxial from the hypaxial muscles of the body wall, and they 
take no part in forming the haemal arches of the caudal region. 
Ventral ribs, on the other hand, always lie internal to the 
hypaxial muscles, and directly external to the peritoneal lining 
of the coelom, and they usually contribute to the formation of 
