166 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
specimens we dissected, while a sixth was less constant. When 
present, this last was connected with either the anterior or the 
posterior efferent arteries of the sixth arch or with both. Occasion¬ 
ally, as shown in the figure, it failed to reach the vessels of any 
arch. The variations in this respect were often unsymmetrical. 
Judging from the descriptions and figures given by various 
authors, commissural arteries of the fourth arch occur in all sharks. 1 
The only exception to this statement is the observation made by 
de Blainville (’ll, p. 117) that in iSqualuspelegrinus the coronary 
arteries probably come from the efferent arteries of the posterior 
arch, presumably the sixth; but as the origin of these vessels Avas 
not exactly determined, this may have been a mistaken surmise. 
From the evidence of previous figures and descriptions, commis¬ 
sural arteries of the fifth arch occur in Mustelus (Hyrtl, ’72, p. 271 ; 
Parker, ’87, p. 697), in Zygaena (Hyrtl, ’72, p. 271), and in Scyl- 
lium (Hyrtl, ’72, p. 267 ; Marshall and Hurst, ’92, p. 242), where, 
however, they have been called by Hyrtl, Arteria nutriens recur- 
rens (brancliialis). Commissural arteries of the sixth arch, such 
as occasionally occur in Carcharias, have been figured only in 
Zygaena by Hyrtl (’72, Tab. 3, fig. 2). 
The median hypobrancliial artery (PI. 1, fig. 1, Wbrn. m.) in 
Carcharias is formed on the ventral side of the ventral aorta by the 
union of the right and left fourth commissural arteries. This 
A r essel has no anterior branch such as Hyrtl (’72, p. 271) has 
described in Zygaena under the name of Arteria thyreoidea impar, 
but extends entirely in a posterior direction, and, after giving off 
what Hyrtl (’72, p. 269) has called the epigastric branch ( e'gci .), 
becomes the ventral coronary (cor. v.). A dorsal coronary artery 
(PL 2 , fig. 4, cor. d.) is formed on the dorsal side of the v r entral 
aorta by a corresponding union of the right and left fifth commis¬ 
sural arteries, supplemented by the sixth when they are present. 
The trunk thus formed lies so near the heart that it may be called 
the dorsal coronary artery (cor. d .), though it might without impro¬ 
priety be regarded as in part a median hypobranchial. 
Much the same condition as that found in Carcharias has been 
described for Zygaena (Hyrtl, ’72, p. 271, Taf. 3, fig. 2) and Mus¬ 
telus (Parker, ’87, PI. 34, fig. 2), except that in these fishes the 
1 They are found in Scyllium according to Hyrtl (’72, p. 267) and to Marshall and 
Hurst ('92, p. 242); in Mustelus according to Hyrtl (’72, p. 271) and Parker (’87, p. 697); 
and in Squatina, Acantliias, and Zygaena according to Hyrtl (’72, p. 268-269 and 271). 
