52 MH. K. H. BURNE ON THE AORTIC-AECH 



the member of the family least modified in this respect, to make 

 out that the swim-bladder is supplied from the fourth aortic 

 arch*. In Proiopterus f this is no longer possible, as the efi'erent 

 branchial vessels have become fused on either side into a common 

 truut, from the p osterior face of which the pulmonary artery 

 arises. The branchial compression is still more advanced in 

 Lepidosiren J, the remaining member of the family ; so much so 

 indeed, that the pulmonary artery apparently takes its origin 

 from the third aortic arch. "Whether this is really the case I 

 must leave an open question, although the great resemblance 

 betvpeen the aortic arches in this fish and in some of the lower 

 Amphibia § would incline one to think not. 



In the two Granoids Polypterus and Amia ]] the pulmonary 

 artery takes its origin, according to Boas %^ as a large branch of 

 the fourth eflferent branchial vessel. The main trunk of this 

 vessel, after giving off the pulmonary artery, passes on in a 

 reduced condition, and joins the third efferent branchial vessel. 

 Thus Boas regards the pulmonary artery of these fish as a deri- 

 vative of the fourth branchial aortic arch alone. 



In contradiction to this, it appears, from the figure of the 

 aortic arches of Amia given by Earasay Wright, that the third 

 aortic arch is also involved in the formation of the pulmonary 

 artery. This effect is produced by the connection between the 

 third and fourth eflPerent branchial vessels being represented 

 as a branch of the third, and not a continuation of the fourth 

 efferent branchial vessel. A very slight alteration in the drawing 

 is enough to accomplish this ; for if the connection in question 

 is drawn sloping from the third branchial vessel towards the 

 middle line, it appears to be part of the third arch ; if away 

 from the middle line ever so slightly, it would be called a conti- 

 nuation of the fourth arch. 



* Boas, Morph. Jahrb. Bel. vi. 1880, p. 321, "Ueber Herz und Artei-ien- 

 bogen bei Ceraiodus und Frotopteriis." 



t Parker (W. N.), "On the Anatomy and Physiology oi Protopterus amiec- 

 tens," Trans. Eoy. Irish Acad. toI. xxx., in which paper other references will 

 also be found. 



\ Hyrtl, Lepidosiren paradoxa, and Bisehoff, " Sur le Lepidosiren paradoxa," 

 Ann. Sci. Nat. (Zool.) toI. xiv. 1840, p. 116. 



§ Bisehoff, I. c. 



II Johannes Miiller, " Beitriige zu Bau uudGrenzen der Ganoiden," Abhandl. 

 Akad. Wiss. Berlin, 1844. p. 117; Boas, Moi-ph. Jahrb. Bd. vi. p. 321. 



«1 Boas, Morph. Jahrb. Bd. vi. pp. 342 & 351. 



