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plies the external gill of the fish, there being, according to Budgett, 

 directly continuous with the efferent artery of the arch; a fact my 

 sections were not carried far enough back to permit me to control. 

 When the efferent hyoidean artery, running forward internal to the 

 operculum, reaches a point immediately ventral to the operculo - hyo- 

 mandibular articulation, it gives off' a stout branch. This branch runs 

 downward and forward, a certain distance, along the hind edge of the 

 hyomandibular — there lying slightly dorsal to the afferent hyoidean 

 artery — and then passes to the external surface to the hyomandi- 

 bular, where it is easily traced to the region of the hyomandibulo- 

 stylohyal articulation. The branch thus has the general course of a 

 commissural branch between the hyoidean and mandibular aortic arches, 

 and is probably the homologue of the commissure between those vessels 

 shown in my diagrams of elasmobranchs, of Lepidosteus and of Aci- 

 penser, in the work above referred to as now in press. Budgett 

 (1901) says that this branch "runs parallel with the afferent (hyoidean) 

 artery", and that its presence "is suggestive, as indicating the position 

 of the pseudobranch, corresponding to the pseudobranch of Acipenser, 

 of which in Polypterus there is no further trace." The expression 

 "parallel with the afferent (hyoidean) artery" is true only for a limited 

 portion of the branch, and the terminal portion of its course, external 

 to the hyomandibular, would seem to preclude its having had any re- 

 lation to an aborted hyoidean pseudobranch, which is what Budgett 

 doubtless intended to suggest. 



After giving off this commissural brail ch, the efferent hyoidean 

 artery passes internal to the hyomandibular, and then inward posterior 

 to the spiracular canal, where it meets and fuses with the efferent 

 artery of the first branchial arch, as shown in Budgett's (1902) 

 figures. This point of fusion lies, in my embryo, dorso - internal to 

 the outer end of the so-called inferior process of the parasphenoid 

 (Traquair, 1870), and dorsal also to the anterior end of the thymus 

 (Pollard, 1892); which latter organ is transversed by the efferent 

 artery of the first branchial arch on its way upward to join the eff"erent 

 hyoidean artery. The thymus extends from here, internal to the hind 

 edge of the spiracular canal, backward to the point where the efferent 

 artery of the second branchial arch curves inward to join the dorsal 

 aorta; lying always along the roof of the pharynx, largely ventro- 

 mesial to the dorsal ends of the branchial arches, instead of in its 

 usual teleostean position, dorsal to those arches and along the dorsal 

 portion of a prolongation of the pharyngeal cavity that extends dor- 

 sally between the side wall of the skull and the gill cover. This po- 



