STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 8oi 



by independent outgrowths of the basal parts of the haemal pro- 

 cesses. 



In Elasmobranchii a still further divergence from the primi- 

 tive 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, but affecting 

 only their ends, is observable in Lcpidosteus}. 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 processes of the tail far less clear than in other types, 

 and further proof is required before such homology can be con- 

 sidered as definitely established. 



This is not the place to enter into the obscure question as to 

 how far the ribs of the Amphibia and Amniota are homologous 

 with those of Fishes. It is to be remarked, however, that the 

 ribs of the Urodela (i) occupy the same position in relation to 

 the muscles as the Elasmobranch ribs, (2) that they are con- 

 nected with the neural arches, and (3) that they coexist in the 

 tail with the hremal arches, and seem, therefore, to be as differ- 

 ent as possible from the ribs of the Dipnoi. 



PART IV. The skeleton of the ventral lobe of the tail fin, and its 

 bearing on the nature of the tail fin of the various types of Pisces. 



In the embryos or larvae of all the Elasmobranchii, Ganoidei, 

 and Teleostei which have up to this time been studied, the un- 

 paired fins arise as median longitudinal folds of the integument 

 on the dorsal and ventral sides of the body, which meet at the 

 apex of the tail. The tail at first is symmetrical, having a form 

 which has been called diphycercal or protocercal. At a later 

 stage, usually, though not always, parts of these fins atrophy, 

 while other parts undergo a special development and constitute 

 the permanent unpaired fins. 



Since the majority of existing as well as extinct Fishes are 

 provided with discontinuous fins, those forms, such as the Eel 

 (Anguilla\ in which the fins are continuous, have probably re- 



