94 r. M. BALFOUR AND W. N. PARKER. 



are grave difficulties in the way of comparing the vertebrae 

 of Lepidosteus with those of the Urodela^ in the fact that iu 

 Lepidosteus the intervertebral cartilages originate from the 

 bases of the arches, while in the Urodela they are stated by 

 Gotte to be thickenings of a special cartilaginous investment 

 of the notochord, which would seem to be homologous with 

 that cartilaginous sheath which is placed in Elasmobranchii 

 and Dipnoi within the membrana elastica externa. On the 

 other handj the development of the vertebrae of Lepidosteus 

 is shown to I'esemble in most features that of Teleostei, from 

 which it mainly differs in the presence of intervertebral 

 cartilaginous rings. 



In the second section, devoted to the homologies of the 

 ribs of Pisces, the conclusions arrived at are summed up as 

 follows : 



The results of the authors' researches appear to leave two 

 alternatives as to the ribs of fishes. One of these, which 

 may be called Gottes view, may be thus stated : — The 

 haemal arches are homologous throughout the Pisces ; in 

 Teleostei, Ganoidei and Dipnoi, the ribs, placed on the inner 

 face of the body wall, are serially homologous with the 

 ventral parts of the haemal arches of the tail ; in Elasmo- 

 branchii, on the other hand, the ribs are neither serially 

 homologous with the haemal arches of the tail, nor homo- 

 logous with the ribs of Teleostei and Ganoidei, but are out- 

 growths of the haemal processes into the space between the 

 dorso-lateral and ventro-lateral muscles, and outgrowths 

 which may perhaps have their homologies in Teleostei and 

 Ganoids in certain accessory processes of the vertebrae. 



The other view, which the authors are inclined to adopt, 

 is as follows : — The Teleostei, Ganoidei, Dipnoi and Elasmo- 

 branchii are provided with homologous haemal arches, which 

 are formed by the coalescence below the caudal vein of 

 simple prolongations of the primitive haemal processes of the 

 embryo. The canal enclosed by the haemal arches can be 

 demonstrated embryologically to be the aborted body cavity. 



In the region of the trunk the haemal processes and their 

 prolongations behave somewhat differently in the different 

 types. In Ganoids and Dipnoi, in which the most primitive 

 arrangement is probably retained, the ribs are attached to 

 the haemal processes, and are placed immediately without the 

 peritoneal membrane, at the insertion of the intermuscular 

 septa. These ribs are in many instances (Lepidosteus, 

 Acipenser), and very probably in all, developed continuously 

 with the haemal processes, and become subsequently seg- 

 mented from them. They are serially homologous with the 



