1268 DR. ERIK A. STENSIO : NOTES 



Being forced to shift their origins still farther backwards by the 

 continued growth of the eye-bulbs, the recti externi muscles may 

 be imagined to have pushed backwai'ds into the posterior wall of 

 the transvei'sal canal, forming a fossa there opening in front 

 into the transverse canal. Then, perhaps chiefly owing to the 

 influence of the other recti muscles, a widening of the transverse 

 canal forward may be supposed to have taken place at the 

 expense of the bone part between this canal and the vertically 

 ascending canal for the internal carotid arteries. Finall}', the 

 transverse cana,l, by the influence of the recti muscles, grew 

 wider downwards as well, the floor of the canal belonging to the 

 primordial neurocranium at the same time becoming much 

 reduced, and finally perhaps forming only a membrane on the 

 dorsal surface of the parasphenoid. The recti inferiores, po;;- 

 teriores, and superiores can after this have been able more or less 

 easily to break through the membrane with their origins in 

 seeking a firm attachment on the paras|)henoid, and if this had 

 happened a ventral myodomic compartment Avould ha,ve arisen. 

 That a myodome arisen in the way sketched here from the con- 

 ditions in the Rhipidistids must lack homologues of the lateral 

 portions of the prepituitaiy parts of the myodome in Birgeria, 

 Amia, and Teleosts, as the myodome in the Coslacanthids actually 

 does, is evident from what has been put forward above concerning 

 the Rhipidistids.'" 



" In this connection finally it ought also to be pointed out that 

 the occurrence of a myodome \\\ the Ctelacanthids shows with full 

 certainty that a myodome can easily develop, and that the myo- 

 dome in several grou])S of fishes has been developed independently.'' 



" The myodome in the Ccelacanthids, if it haa no ventral 

 compartment, would, as is easily understood, correspond fairly 

 well to the myodome in S. ornahis if the latter were extended 

 forward by reduction of the larger posterior p^u■t of the bone- 

 pa.rt hs'ph.''' 



If we summarize here the results obtained by the investigation 

 of the Coelacanthids with regard to the basipterygoid process, we 

 find the following. The basipterygoid process ©f the primitive 

 ancestors of the Coelacanthids must have been rather high, 

 extending upwards along the lateral side of the primordial 

 neurocranium as in Dicti/onosteus, i\nd them eta pterA'goid probably 

 articulated against the upper parts of this process. Through 

 reduction of the ventral parts of the process the conditions 

 which occur in Dijyloce^'cides seem to have arisen, and from these 

 the evolution towards the post-Devonian Coelacanthids ought to 

 have proceeded in such a way that the parts of the cranial basis 

 below the remains of the basipterygoid process grew very deep, 

 while at the same time the height of the parts dorsally of this 

 process decreased. 



In Diploce7-cides the sphenoid, like that of the Rhipidistids, 

 comj^rises paired orbitosphenoid and alisphenoid components ar<l 

 an unpaired basisphenoid component. In the post-Devonian 



