STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 783 



ligament (Plate 41, fig. 70, /./.), but there is as yet no trace of a 

 junction between the neural arches of the two sides in the space 

 between the longitudinal ligament and the spinal cord. 



The basal parts of the neural arches of the two sides are 

 united dorsally by a thin cartilaginous layer resting on the 

 sheath of the notochord, but they are not united ventrally with 

 the haemal arches. 



The haemal processes in the trunk are much more prominent 

 than in the preceding stage, and their bases are united ventrally 

 by a tolerably thick layer of cartilage. In the trunk they are 

 continuous with the so-called ribs of the adult (Plate 41, fig. 70) ; 

 but in order to study the nature of these ribs it is necessary to 

 trace the modifications undergone by the haemal arches in pass- 

 ing from the tail to the trunk. 



It will be remembered that at an earlier stage the haemal 

 arches in the region of the tail-fin were fully formed, and that 

 through the anterior part of the caudal region the haemal pro- 

 cesses were far advanced in development, and just in front of 

 the caudal fin had actually met below the caudal vein. 



The mode of development of the haemal arches in the tail as 

 unjointed cartilaginous bars investing the caudal arteries and 

 veins is so similar to that of the caudal haemal arches of 

 Elasmobranchii, that it appears to us impossible to doubt their 

 identity in the two groups 1 . 



The changes which have taken place by this stage with 

 reference to the haemal arches of the tail are not very con- 

 siderable. 



In the case of a few more vertebrae the haemal processes 



1 Gegenbaur (No. 6) takes a different view on this subject, as is clear from the 

 following passage in this memoir (pp. 369370): "Each vertebra of Lcpidosteus 

 thus consists of a section of the notochord, and of the cartilaginous tissue surrounding 

 its sheath, which gives origin to the upper arches for the whole length of the vertebral 

 column, and in the caudal region to that of the lower arches also. The latter do not 

 however complete the enclosure of a lower canal, but this is effected by special independent 

 elements, which are to be interpreted as homologues of the ribs." (The italics are 

 ours.) While we fully accept the homology between the ribs and the lower elements 

 of the hremal arches of the tail, the view expressed in the italicised section, to the 

 effect that the lower parts of the caudal arches are not true haemal arches but are 

 independently formed elements, is entirely opposed to our observations, and has we 

 believe only arisen from the fact that Gegenbaur had not the young larvae to work 

 with by which alone this question could be settled. 



