. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 144 MORPHOLOGY OF THE ORCAA'S OF VERTEBRATES. the vertebrae forward, it is seen that the transverse process of the sacral vertebra, considerably enlarged, supports the pelvic arch, while in the presacral vertebrae these same transverse processes bear short articulated elements, — the ribs. It follows from this (i j that the amphibian ribs are not equivalent to the haemal processes in these animals, and (2) that they are struc- tures different from the ribs of fishes. This view is farther substantiated by the conditions which
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 144 MORPHOLOGY OF THE ORCAA'S OF VERTEBRATES. the vertebrae forward, it is seen that the transverse process of the sacral vertebra, considerably enlarged, supports the pelvic arch, while in the presacral vertebrae these same transverse processes bear short articulated elements, — the ribs. It follows from this (i j that the amphibian ribs are not equivalent to the haemal processes in these animals, and (2) that they are struc- tures different from the ribs of fishes. This view is farther substantiated by the conditions which ob- tain in the ganoid Polypteriis, where both types of ribs, those of fishes and those of the higher vertebrates, occur in the same segment, the latter lying in the connective tissue between the epi- and hypaxial muscular systems. The ribs of the amniotes are clearly homologous with those of the amphibia. They are intersegmental in position, and arise by a condrification and more or less complete ossification of part of the myo- commatous tissue, a mode of development which readily explains their frequent ex- tension to the ventral surface. In the fishes the ribs (sometimes lacking, as in some plectognaths and lo- phobranchs) are usually slender, and are frequentl}" firmly united to the vertebral centra; or, again, they may be raovably articulated to short 'basal stumps.' In many physostomous fishes some of the anterior ribs are modi- fied to give rise to a chain of bones connecting the air-bladder with the ear. Besides these ribs, there frequentl)- occur in fishes slender bones in the fleshy portions, the homologies of which remain to be ascertained. Possibly some of them mav represent the ribs of the higher forms. These epimerals, epi- centrals, and epipleurals, as they are called, are stated to be without a cartilage stage. The ribs of the elasmobranchs are small and cartilaginous, and are more or less intimately united with the vertebral Fig. 152. Section
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