514 



the ventral half or two thirds of the anterior commissural vessel is 

 found, and it partly encircles the ventral end of the sixth gill cleft. 



In the hyoidean arch there is but a single efferent vessel, and this 

 vessel not only has the relative position, in its arch, of the posterior 

 one of the two efferent arteries in each of the branchial arches, but, 

 like those posterior branchial arteries it is connected by commissure 

 around the ventral end of the intervening cleft with the anterior 

 efferent artery in the next succeeding arch, that is, in this case, with 

 the anterior efferent artery in the first branchial arch. Anteriorly, 

 another commissure connects the ventral end of the efferent hyoidean 

 artery with the ventral end of what is certainly, as in other elasmo- 

 branchs, the persisting remnant of the primary mandibular aortic arch. 



From each one of the efferent loops that encircle the ventral ends 

 of the gill clefts, excepting only the sixth cleft, a vessel arises. The 

 vessel that arises from the loop that connects the hyoidean and first 

 branchial arches anastomoses with the loop that connects the next two 

 succeeding arches, the two loops and the connecting branch forming a 

 nearly straight and continuous ventral commissure. The vessel that 

 arises from the loop that connects the first and second branchial 

 arches runs mesially until it reaches the lateral surface of the truncus 

 arteriosus, where it turns backward and unites with the vessel from 

 the next succeeding loop. The vessel so formed continues backward 

 a short distance and then unites with its fellow of the opposite side 

 to form a median artery which lies ventral to the truncus arteriosus. 

 This median artery soon separates again into two parts, a coronary 

 artery on either side, each of which runs backward to the heart. 

 The vessel that arises from the efferent loop related to the fourth 

 gill cleft runs mesially and unites with its fellow of the opposite side 

 to form a median vessel that lies dorsal to the truncus arteriosus. 

 This median vessel is joined by the vessel that arises, on either side, 

 from the efferent loop related to the fifth gill cleft, and the artery so 

 formed then separates into two parts each of which encircles the 

 truncus arteriosus and joins the related coronary artery. These ventral 

 commissures and loops of the efferent branchial arteries are not shown 

 by Ayers, but a coronary artery is shown by him and is said to be 

 a branch of the ventral aorta. Ayers further says (p. 194), "From 

 the anterior edges of the anterior innominate arteries, equi- distant 

 from the median line, arise two small vessels, which, passing forwards, 

 supply the muscles in the ventral wall of the throat." No such vessels 

 could be found in either of my specimens, and it would seem as if 

 they must be represented in the afferent mandibular artery and its 



