STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
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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, 
however, the haemal canal became prolonged forwards by the formation 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. 
Fig. 2. 
Transverse section through the ventral part of the notochord, and adjoining structures of an advanced 
Scyllium embryo at the root of the tail. 
Vb., cartilaginous sheath of the notochord; ha., hsemal process; r.p., process to which the rib is 
articulated ; m.el., membrana elastica externa j ch., notochord; ao., aorta ; V.cau., caudal vein. 
In some Elasmobranchii, as shown in the accompanying woodcut (fig. 2), in the 
transitional vertebrae between the trunk and the tail, the ribs are supported by lateral 
3 F 2 
