283 



In the dorsal (proximal) half of the fourth arch there is but a 

 single hemibraach, the gill opening between this arch and the fifth 

 arch only extending to the dorso-posterior ends of the ceratobranchials 

 of the arches. This single hemibranch is, in larvae, drained by a 

 loop of the fourth efferent artery which begins near the hind (distal) 

 end of the epibranchial of the arch, and from there runs upward and 

 forward along the bases of the gill filaments to fall into the main 

 artery again at the upper end of the gill. In the adult, according 

 to Allen's drawings, this dorsal connection of this gill artery with 

 the main one does not exist, the gill artery thus there being a branch 

 of the main one and not a loop. Slightly dorsal to the ventral origin 

 of the loop the coronary artery is given off. This artery was not 

 traced in sections, but, according to Allen's drawings, it reaches the 

 heart from the rear, giving off just before it reaches that organ, 

 branches to what are apparently pharyngo-claviculares extern us and 

 internus muscles. Dorsal to the coronary artery, branches are sent, 

 in my larvae, to muscles related to the fourth and fifth arches; these 

 small arteries apparently not injecting in the adult, for they are not 

 shown in Allen's drawings. 



On the fifth arch there are gill rakers, but no gill filaments. 



In addition to these four efferent branchial arteries, there is an 

 efferent mandibular artery and a functionless efferent hyoidean artery. 

 The latter artery has become a branch of the external carotid and 

 will be described, in connection with that artery, as its hyo-opercularis 

 branch. The efferent mandibular artery is the efferent artery of the 

 pseudobranch, and is called by Allen, as it is generally called in 

 selachians, the anterior, or internal carotid. This efferent pseudo- 

 branchial artery, after its origin from the pseudobranch, runs forward 

 external to the skull, and joins the internal carotid in the region of 

 the ascending process of the parasphenoid. From there the two 

 arteries run forward, lying close together, until the efferent pseudo- 

 branchial artery falls into the internal carotid, the detailed description 

 of this part of its course being given below when describing the 

 internal carotid. The efferent pseudobranchial artery gives off but a 

 single branch in its entire course, the arteria ophthalmica magna, 

 which will be described when describing the course of the artery. 



Anterior to the point where the first efferent branchial artery joins 

 the lateral dorsal aorta the latter artery becomes the common carotid ; 

 the carotid being said, in the terminology ordinarily employed, to 

 arise from the efferent artery of the first branchial arch as that 

 artery curves inward and mesially to fall into the anterior end of the 

 medial dorsal aorta. The common carotid, considered by Allen as 



