487 



a lateral dorsal aorta on either side, these two arteries lying at first close 

 together, enveloped in a common sheath, and running forward along 

 the median hne of the ventral surface of the skull immediately beneath 

 (dorsal to) a stout muscle-mass formed by two muscles, one on either 

 side, that must each be the M, subspinalis of Vetter's (1874) de- 

 scriptions of Acanthias. Having reached the anterior edge of this 

 muscle, each lateral dorsal aorta turns antero-laterally and having 

 passed the lateral edge of the base of the skull gives off, at nearly 

 a right angle, the external carotid, which artery has a much larger 

 caliber than the lateral dorsal aorta posterior to this point. After 



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Fig. 2. Diagrammatic representation of the arteries in Heptanchus (cf. Fig. 1). 



giving off this external carotid artery the lateral dorsal aorta imme- 

 diately acquires a caliber equal to that of the external carotid, and 

 making a short curve postero-laterally receives, at nearly a right angle, 

 the efferent hyoidean artery, which artery is of still larger cahber 

 than the lateral dorsal aorta at this point. The lateral dorsal aorta 

 then turns sharply dorso-antero-mesially, in the direction prolonged 

 of the efferent hyoidean artery, and acquiring the caliber of the latter 

 artery appears as a direct anterior continuation of it. Because of the 

 greatly differing calibers of the three sections of the lateral dorsal 

 aorta here under consideration, they do not appear, in dissections, 

 as parts of a single continuous vessel, the efferent hyoidean artery 



