RIBS. 561 



Amphibia, on the level of division between the dorso-lateral and 

 ventro-lateral divisions of the muscle-plates. This does not 

 appear to hold true for either Ganoidei or Teleostei. In 

 Teleostei they are entirely below the muscles along the lines of 

 the intermuscular septa, and this is partially true for Ganoidei, 

 though not wholly so in Lepidosteus. They may be attached 

 either to the haemal (Pisces) or neural (Amphibia and Amniota) 

 arches. The connective tissue from which they are formed is 

 continuous with the processes of the vertebrae to which they are 

 attached ; but the conversion of the tissue into cartilage takes 

 place more or less independently of that of the arches, although 

 in many cases the cartilage of the two becomes continuous, the 

 separation of the ribs being then effected by a subsequent 

 process of segmentation (Pick, No. 431). It is possible that the 

 ribs of Pisces may not be homologous with those of Amphibia 

 and the Amniota, but till the reverse can be proved it is more 

 convenient to assume that the ribs are homologous structures 

 throughout the vertebrate series. 



In Elasmobranchii the ribs are relatively of less importance in the 

 adult than in the embryo. By a careful examination of their early develop- 

 ment, I have satisfied myself that the differentiation of the ribs is indepen- 

 dent of that of the haemal processes to which they are attached, although 

 the differentiation proceeds in such a manner that, when both are converted 

 into cartilage, they are quite continuous. Subsequently the ribs become 

 segmented off from the haemal processes. At the junction of the tail and 

 trunk, where the haemal processes commence to be ventrally prolonged, 

 eventually to unite in the region of the tail below the caudal vein, the ribs 

 are attached to short processes which spring from the sides of the haemal 

 arches (fig. 317). The ventral haemal arches of these" fishes are therefore 

 clearly in no part formed by the ribs. 



In Ganoidei and Teleostei there is very great difficulty in determining 

 the homologies of the ribs. 



In the cartilaginous Ganoidei there are well developed rib-like struc- 

 tures, which might be regarded as homologous with Elasmobranch ribs, 

 and indeed probably are so ; but at the same time their relations are in 

 some respects very different from those of Elasmobranch ribs in the caudal 

 region. In Ganoids the ribs, in approaching the tail, become shorter and then 

 fuse with the ends of the haemal processes, and finally in the caudal region 

 form together with the haemal arches a closed haemal canal which super- 

 ficially resembles that in Elasmobranchii. 



In Lepidosteus and Amia, especially the former, the same phenomenon 

 is still more marked ; and in Lepidosteus it is easy, in passing backwards, 



B. III. 36 



