STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 795 



(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 vcntro-lateral muscles (woodcut, fig. i, rb.}, 



FIG. i. 



...sp.c 



... ft a 



II. 



Diagrammatic section through the trunk of an advanced embryo of Scyllium, to shew 



the position of the ribs. 



ao., aorta; c. sh. , cartilaginous notochordal sheath ; cv., cardinal vein ; /if., hremal 

 process; k., kidney; I.s., ligamentum longitudinale superius ; m.cl., membrana 

 elastica externa; na., neural arch; no., notochord ; //., lateral line; rb., rib; 

 sp.c., spinal cord. 



while in Lepidosteus and other Ganoids they immediately girth- 

 the body-cavity. 



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 haemal arches throughout the Pisces. 



512 



