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dorsal aorta separates into its two lateral portions, lies wholly within 

 the cartilage of the skull, not touching the notochord at any point, 

 and it opens anteriorly into the pituitary fossa. The canal in Polypterus 

 ends, anteriorly, in so far as its relations to the arteries are concerned, 

 at a point corresponding to that in which it begins in Chlamydo- 

 selachus, and while it traverses, according to Budgett (1902), a short 

 portion of the occipital cartilage, it is always bounded dorsally by the 

 notochord. But in my specimen there is no slightest trace of the 

 cartilaginous bridge said by Budgett to enclose this canal in his 

 30 mm specimen ; the canal being enclosed, its entire length, by the 

 notochord above, and by bone ventrally and on either side. And this 

 enclosing bone has every appearance of being wholly and purely of 

 membrane origin. As Budgett's specimen and my own are both 

 Polypteri senegali, this difference can not be a specific one, and hence 

 must be either an individual difference, or a difference due to age. 

 In the adult Polypterus bichir this bridge must have been also wholly 

 wanting in the specimens examined by Bridge and Pollard; for 

 according to Bridge the aortal canal is bounded ventrally, its entire 

 length, by the parasphenoid ; and Pollard so shows it in his Fig. 12. 

 In Pollard's figure 23 it is however shown entirely enclosed in the 

 basioccipital, an a I so find it in a large adult Polypterus bichir that I 

 have cut in median vertical section. In Pollard's figure, the enclosing 

 part of the basioccipital would seem not to have been preformed in 

 cartilage, as I find it in my young specimen of P. senegalus, while in 

 my adult P. bichir, it would seem, in superficial examination, to have 

 been preformed in cartilage: this seeming to indicate that there may 

 be individual differences in this respect in these fishes. 



While tracing the vessels in this region of Polypterus I have been 

 necessarily led to consider somewhat the question of a myodome, and 

 while this subject can not be properly discussed here, it can be briefly 

 stated that Polypterus certainly presents a primitive condition of that 

 canal. In a work now in press (Allis, 1) I have endeavoured to show 

 that the myodome of Amia and teleosts is not derived, as Sagemehl 

 (1883) concluded, from the conditions shown in Lepidosteus, but from 

 some such conditions as are presented in Polypterus. But it was 

 necessary, in arriving at this conclusion, to assume that there was in 

 Polypterus, as there is in Amia and teleosts, a pituitary vein that 

 arose in the pituitary fossa of the fish and from there passed directly 

 into the orbit. That the vein must exist in some form was evident, 

 although it had never been described or shown in figures of this region. 

 But that it passed into the orbit, to there join the jugular, was not 



Aiiat. Anz. XXXIII. AufsStze. 15 



