OSTEOLOGY, ETC., OF SYNGNATHUS PECKIANUS. 643 



chordals must be placed in one and the same category."^ Still 

 later, he speaks for the presence of three prseoral arches, which 

 he names epipterygoid, ethmo-palatine, and prorhinal.^ 



Marshall's view is that only two arches are represented in 

 the prsBoral region, namely, the ethmo-palatine, or lachrymal, 

 and the olfactory.^ 



Upon what grounds is it possible to base conclusions as to 

 this point ? It seems to me that before we can come to any 

 definite conclusion as to whether a structure is really a visceral 

 arch, we must be able to show the presence or rudiments of an 

 arch, a cleft, a nerve supplying that arch, and a head-cavity. 

 Let us now apply these tests. 



In front of the trigeminal nerve which supplies the man- 

 dibular arch, we have the olfactory, optic, oculomotor, pathe- 

 ticus ; and the abducens may also be enumerated here, as it has 

 not yet been accounted for. It has been shown by Marshall 

 and others that of these the optic, paiheticus, and abducens 

 are probably not entitled to be ranked in the same category as 

 the other cranial nerves, leaving only the oculomotories (iii) 

 and olfactory (i) to be accounted for. The former is referable 

 to the region of the ethmo-palatine cartilage, and we have also 

 a cleft, the lachrymal, and a head-cavity, the praemandibular, 

 the walls of which, according to Marshall,* become transformed 

 with the superior, inferior, and internal recti, and inferior 

 oblique muscles of the eye, to which the third cranial nerve acts 

 as a supply. 



All the necessary parts are present, then, for forming a firm 

 basis on which the conclusion as to the validity of the ethmo- 

 palatine or lachrymal arch may be vested. The olfactory arch 

 is, however, much more complicated. 



1 Parker and Bettany. 'On tlie Morpliolotry of the Skull,' London, 1877. 



* W. K. Parker, " Oti the Evolution of the Vertebrates," Huuteriau Lec- 

 tures, 'Nature,' vol. xx, 187'J. 



^ A. Milnes Marshall, "The Morphology of the Vertebrate Olfactory Organ," 

 * Quart. Journ. Micr. Sci.,' vol. xix, 1879, 



•* A. Millies Marshall, "On the Head-cavities and associated Nerves of 

 Elasmobranchs," ' Quart. Journ. Mic. Sci.,' vol. xxi, 18S1. 



