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then have travelled backward along the internal carotid to its actual 

 position ; a not impossible but improbable supposition. One is accord- 

 ingly almost forced to the conclusion that, if the organ is an aborted 

 mandibular pseudobranch, it must, before its abortion, have acquired 

 an important and secondarily established connection with the internal 

 carotid, this connection probably lying posterior to the adductor arcus 

 palatini muscle. But if this structure in Ameiurus be considered to 

 be the homologue of the thymus of Polypterus, whatever the origin 

 of that organ may have been, none of these objections apply. The 

 thymus of Polypterus is supplied (Allis, 3) by an artery that has its 

 origin from the efferent hyoidean artery close to the base of the common 

 carotid, and it already has a position considerably anterior to that 

 that is usual for this organ; thus suggesting (Allis, 3), that it may 

 represent a portion of the organ developed in relation to anterior 

 arches only. A slight change in its position could give rise to the 

 organ in Ameiurus. 



In comparing the positions of the thymus of Polypterus and the 

 so-called pseudobranch of Ameiurus, it is to be remarked that the 

 adductor arcus palatini of Ameiurus seems wholly wanting in Poly- 

 pterus; for that it can be represented in what Pollard ('92) describes 

 as the pterygoid division of the adductor mandibulae seems impossible 

 because of the innervation of this latter muscle, which is said to be 

 by a branch of the trigeminus instead of by a branch of the facialis. 

 And that it can have been developed from a muscle homologous with 

 the retractor hyomandibularis of Pollard's descriptions of Polypterus, 

 which muscle is properly innervated by the facialis, seems improbable 

 because that muscle is evidently represented in the so-called adductor 

 hyomandibularis of Ameiurus (Mc Muerich, '84); the latter muscle being 

 a retractor, or levator, rather than an adductor. Of the other muscles 

 described by Pollard, the protractor hyomandibularis of Polypterus 

 is evidently the dilatator operculi of Ameiurus; and the levator maxillae 

 superioris of the one is the levator arcus palatini of the other. The 

 important posterior division of the adductor arcus palatini of Ameiurus 

 would thus seem to be wholly wanting in Polypterus; as is also the 

 anterior division of the same muscle. This latter muscle, it may be 

 stated, is distinctly double in my sections, one portion going to the 

 dorsal and the other to the ventral edge of the palatine; the two 

 muscles thus being capable of rotating that bone somewhat, and, 

 through its maxillary process, acting on the tentacle. 



In Amia (Allis, '97), as in Polypterus, the two little muscles 

 that represent the anterior division of McMurrich's descriptions of 



