1881.] On the Structure and Development of Lepidosteus. 117 



tives as to the ribs of fishes. One of these, which may be called 

 Gotte's view, may be thus stated: — The hsemal arches are homolo- 

 gous 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 homologous with the ribs of 

 Teleostei and Ganoidei, bat are outgrowths 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 Elasmobranchii are 

 provided with homologous haemal arches, which are formed by the 

 coalescence below the caudal vein of simple prolongations of the 

 primitive hsemal 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 prolonga- 

 tions 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 heeinal 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 segmented from them. 

 They are serially homologous with the ventral parts of the haemal 

 arches of the tail, which, like them, are in many instances (Ceratodus, 

 Lepidosteus, Polypterus, and to some extent in Amia) segmented off 

 from the basal parts of the haemal arches. 



In Teleostei the ribs have the same position and relations as those 

 in Ganoids and Dipnoi, but their serial homology with the ventral 

 parts of the haemal processes of the tail is often {e.g., the Salmon) 

 obscured by the anterior haemal arches (i.e., those in the posterior part 

 of the trunk) being completed, not by the ribs, but by independent 

 outgrowths of the basal parts of the haemal processes. 



In Elasmobranchii a still further divergence from the primitive 

 arrangement is present. The ribs appear to have passed outwards, 

 along the intermuscular septa, into the muscles ; and are placed between 

 the dorso -lateral and ventro-lateral muscles (a change of position of the 

 ribs of the same nature is observable in Lepidosteus). This change of 

 position, combined probably with the secondary formation of a certain 

 number of anterior haemal arches, similar to those in the Salmon, 

 renders their serial homology with the ventral parts of the haemal pro- 

 cesses of the tail far less clear than in other types ; and further proof 



