808 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



belong, we come to the consideration of the Dipnoi, in which the 

 tail-fin presents problems of more interest and greater difficulty 

 than those we have so far had to deal with. 



The undoubtedly very ancient and primitive character of the 

 Dipnoi has led to the view, implicitly if not definitely stated in 

 most text-books, that their tail-fin retains the character of the 

 piscine tail prior to the formation of the ventral caudal lobe, a 

 stage which is repeated embryologically in the pre-heterocercal 

 condition of the tail in ordinary Fishes. 



Through the want of embryological data, and in the absence 

 of really careful histological examination of the tail of any of 

 the Dipnoi, we are not willing to speak with very great confi- 

 dence as to its nature ; we are nevertheless of the opinion that 

 the facts we can bring forward on this head are sufficient to 

 shew that the tail of the existing Dipnoi is largely aborted, so 

 that it is more or less comparable with that of the Eel. 



We have had opportunities of examining the structure of the 

 tail of Ceratodus and Protopterus in dissected specimens in the 

 Cambridge Museum. The vertebral axis runs to the ends of 

 the tail without shewing any signs of becoming dorsally flexed. 

 At some distance from the end of the tail the fin-rays are sup- 

 ported by what are apparently segmented spinous prolongations 

 of the neural and haemal arches. The dorsal elements are 

 placed above the longitudinal dorsal cord, and occupy therefore 

 the same position as the independent elements of the neural 

 arches of Lepidosteiis. They are therefore to be regarded as 

 homologous with the dorsal fin-supports or interspinous bones 

 of other types. The corresponding ventral elements are there- 

 fore also to be regarded as interspinous bones. 



In view of the fact that the fin-supports, whenever their 

 development has been observed, are found to be formed inde- 

 pendently of the neural and haemal arches, we may fairly assume 

 that this is also true for what we have identified as the inter- 

 spinous elements in the Dipnoi. 



The interspinous elements become gradually shorter as the 

 end of the tail is approached, and it is very difficult from a 

 simple examination of dissected specimens to make out how far 

 any of the posterior fin-rays are supported by the haemal arches 

 only. To this question we shall return, but we may remark 



