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MESSRS. F. M. BALFOUR AND W. N. PARKER ON THE 
and Elasmobranchii might at first sight be taken as a conclusive argument in favour 
of Gotte’s view, that the ribs of Elasmobranchii are not homologous with those of 
Ganoidei. This view is mainly supported by two facts :— 
(1) In the first place, the ribs in Elasmobranchii do not at first sight appear to be 
serially homologous with the ventral parts of the haemal arches of the tail, but would 
rather seem to be lateral offshoots of the haemal processes, while the haemal arches of 
the tail appear to be completed by the coalescence of independent ventral prolongations 
of the haemal processes. 
(2) In the second place, the position of the ribs is different in the two groups. In 
Elasmobranchii they are situated between the dorso-lateral and ventro-lateral muscles 
(woodcut, fig. 1, rb.), while in Lepidosteus and other Ganoids they immediately girth 
the body-cavity. 
Fig. 1. 
7.s 
Diagrammatic section through, the trunk of an advanced embryo of Scyllium, to show the position of 
the ribs. 
ao., aorta; c.sh,, cartilaginous notochordal sheath; cv., cardinal vein; hp., hgemal process; h., kidney; 
l.s., ligamentum longitudinale superius; m.el., membrana elastica externa; na., neural arch; no., 
notochord; ll., lateral line; rb., rib; sp.c., spinal cord. 
There is much, therefore, to be said in favour of Gotte’s view. At the same time, 
there is another possible interpretation of the facts which would admit the homology 
of the ribs as well as of the hsemal arches throughout the Pisces. 
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 conceive 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 
