STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



Let us suppose, to start with, that the primitive arrangement 

 of the parts is more or less nearly that found in Lepidosteus, 

 where we have well-developed ribs in the region of the trunk, 

 girthing the body-cavity, and uniting in the caudal region to 

 form the ventral parts of the haemal arches. It is easy to con- 

 ceive that the ribs in the trunk might somewhat alter their 

 position by passing into the muscles, along the inter-muscular 

 septa, till they come to lie between the dorso-lateral and ventro- 

 lateral muscles, as in Elasmobranchii. Lepidosteus itself affords 

 a proof that such a change in the position of the ribs is not 

 impossible, in that it differs from other Ganoids and from Teleostei 

 in the fact that the free ends of the ribs leave the neighbourhood 

 of the body-cavity and penetrate into the muscles. 



If it be granted that the mere difference in position between 

 the ribs of Ganoids and Elasmobranchii is not of itself sufficient 

 to disprove their homology, let us attempt to picture what would 

 take place at the junction of the trunk and tail in a type in 

 which the ribs had undergone the above change in position. On 

 nearing the tail it may be supposed that the ribs would gradually 

 become shorter, and at the same time alter their position, till 

 finally they shaded off into ordinary haemal processes. If, how- 

 ever, the haemal canal became prolonged forwards by the forma- 

 tion of some additional complete or nearly complete haemal 

 arches, an alteration in the relation of the parts would necessarily 

 take place. Owing to the position of the ribs, these structures 

 could hardly assist in the new formation of the anterior part of 

 the haemal canal, but the continuation forwards of the canal 

 would be effected by prolongations of the haemal processes 

 supporting the ribs. The new arches so formed would naturally 

 be held to be homologous with the haemal arches of the tail, 

 though really not so, while the true nature of the ribs would 

 also be liable to be misinterpreted, in that the ribs would appear 

 to be lateral outgrowths of the haemal processes of a wholly 

 different nature to the ventral parts of the haemal arches of the 

 tail. 



In some Elasmobranchii, as shewn in the accompanying 

 woodcut (fig. 2), in the transitional vertebrae between the trunk 

 and the tail, the ribs are supported by lateral outgrowths of the 

 haemal processes, while the wholly independent prolongations of 



