RIBS 55 



ends of the upper ribs of Polypterus, to Avhicli they probably correspond. 

 There can be little doubt, however, that the more delicate bars which lie 

 ventrally to the larger structures in Polypterus correspond to the ribs of 

 Fishes ("lower ribs ") as described above. It is therefore possible that 

 Polypterus and certain Teleostei possess the representatives of two sets of 

 ribs — the one correspondmg to those of the majority of Pishes, and the 

 other to those of Blasmobranchs, Amphibians, and Amniota (Pig. 40a, E). 



The intermuscular hones present in the m3''ocommata of Teleosts 

 probably correspond simply to ossifications of the septa, and have 

 nothing to do with ribs. 



Elasmobranchii. — The small, cartilaginous ribs of these 

 Fishes arise independently of the vertebral column in the connec- 

 tive tissue of the intermuscular septa, and extend outwards between 

 the dorso-lateral and the ventro-lateral muscles (see Fig. 40 A, 

 D). They are thus not genetically connected with the basal pro- 

 cesses, although they early become united to them by ligament, 

 and therefore do not correspond either to differentiations of 

 the hsemal arches or to transverse processes segmented off 

 from these. 



Amphibia. — The ribs of Amphibians arise in a very similar 

 manner to those of Blasmobranchs, but differ from them in being 

 from the first connected with the neural (dorsal) and not with the 

 haemal (ventral) arches or basal processes. This is due to the 

 phylogenetic upward displacement of the longitudinal septum 

 separating the dorso-lateral from the ventro-lateral muscles. Like 

 those of Elasmobranchii and Amniota, the ribs are situated between 

 these two masses of muscle, but never extend very far laterally or 

 ventrally. 



The ribs of Urodeles are forked at their proximal ends, and 

 articulate with bifurcated transverse processes of the vertebras 

 arising from the arch and centrum respectively : the dorsal part 

 of the transverse process, arising from the arch, is a new acquis- 

 ition. In many cases ribs are present only in the region of the 

 trunk, but occasionally they extend into the base of the tail, 

 where hsemal arches, corresponding to the basal processes of 

 Blasmobranchs, are also present ^ Urodeles therefore possess 

 the representatives of two kinds of ribs, morphologically distinct 

 from one another (comp. Polypterus and Teleostei, p. 54). All 

 the precaudal vertebrae except the first usually bear ribs ; in rare 

 cases (Spelerpes) there are a few ribless lumbar vertebrae. 



In the Anura the ribs are much shorter, and are doubtless 

 degenerated. As a rule, they become fused with the broad trans- 

 verse processes, at the ends of which they are situated ; the anterior 

 ones may sometimes, however, remain distinct (Fig. 34). They are 

 never bifurcated, and no trace of haemal arches exists. 



^ The elements of true ventral arches (basal processes) may also be present all 

 along the trunk in the larva of Salamandra maculosa, and are still more marked 

 in Necturus (Menobranchus). 



