RIBS 



65 



haemal arches, which in Teleosts, as in Elasmobrarichs, are 

 developed from the transverse processes alone (Fig. 50, B). 

 The dorsal ribs take no part in the formation of the haemal arches : 

 towards the posterior part of the trunk they become rudimentary, 

 but may sometimes still be recognised in the tail as lateral 

 processes at the bases of the haemal arches. 



In most Ganoids and in Dipnoans (Fig. 50, A, c) ventral ribs only 

 are present. In Crossopterygians (Polypterus, Figs. 50, E, and 

 51) larger dorsal and smaller ventral ribs occur, so that there are 



-IT 



FIG. 51. ANTERIOR END OF THE VERTEBRAL COLUMN OF POLYPTERUS. From 



the ventral side. 



Ps, parasphenoid ; WK, centra ; / V, first five pairs of dorsal ribs ; ft, ventral 



ribs. 



two pairs of ribs to each body-segment. Dorsal ribs can also be 

 recognised in certain Teleosts (Salmonidae, Clupeoidei) in addition 

 to ventral ribs, and like these, are always preformed in cartilage. 1 

 In many forms, the ventral ribs may undergo reduction, and in 

 Elasmobranchs they are wanting, while dorsal ribs are usually 

 present. 2 In Chimaeroids and many Rays, as is also the case in 

 Cyclostomes, a fibrous band extends outwards from the vertebral 



1 This fact alone is sufficient to distinguish them from the intermuscular 

 bones often present in this region in Teleosts. In addition to these epicentral 

 intermuscular bones, others the epineurals and epipleuralsare situated more 

 dorsally and more ventrally respectively, and all of them are merely ossifications 

 in the septa. 



2 The haemal arches of these Fishes, as well as of Ganoids, Dipnoans, and 

 Amphibians, apparently contain components corresponding to ventrals rib. 



