196 BULLETIN OF THE 



is connected by an anastomosing branch with the second, just as the lat- 

 ter emerges from its arch to enter the roof of the mouth, and thus at 

 least a portion of the blood from the hyoidean demibranch reaches the 

 dorsal aorta ; but as the trunk of the first efferent branchial artery 

 passes out of the hyoid arch it curves forwards along the outer edge of 

 the basis cranii, and runs as far forwards as the middle of the orbital 

 region, where it suddenly curves inwards to a point at one side of the 

 median line, just below the pituitary space, the floor of which it perfo- 

 rates to enter the cranial cavity. This is the first impression produced 

 on laying bare the vessels in Chlamydoselachus, but, as we shall see later 

 on, all of this trunk lying beyond the upper end of the hyoid is foreign 

 to the first efferent artery, whose continuation we are to seek in the 

 branch uniting it with the second, and is simply the trunk of the 

 common carotid artery. 



The commissural branch uniting the hyoidean or first efferent bran- 

 chial artery to tlie second is fully as large as the arteries themselves, 

 and from its manner of union with the second and of its separation 

 from the first efferent branchial makes the conclusion unavoidable that 

 it is in truth the continuation of the trunk of the first efferent branchial, 

 which however fails of an independent union with the dorsal aorta, but 

 in a manner similar to that in which the homologous arteries in Scyl- 

 lium, Acanthias, and Zygsena (Hyrtl, loc. cit., Plates I.-III.) are united, 

 i. e. by the fusion of the latter with the next succeeding branchial 

 artery (see Figs. 10 and 11). This method of fusion is carried to 

 an extreme in the bony fishes, where all the efferent branchials of each 

 side unite to form the single pair of aortic roots (Midler, loc. cit., 

 Plate III. Fig. 13), and is also represented in Chlamydoselachus in the 

 posterior section of the effei'ent branchial system by the fusion of the 

 sixth and fifth efferent branchials to form a single aortic root. Under 

 primitive conditions of the hyoidean gill this anastomotic vessel would 

 take blood from the anterior demibranch of the hyoidean gill sac and 

 the posterior demibranch of the mandibular gill sac, — holobranch of 

 Parker. On account of the reduction which has taken place in the 

 spiracular gill among existing Elasmobranchs, this vessel serves simply 

 to convey the blood from the anterior demibranch of the first gill sac 

 into the efferent artery of the first holobranch. 



As in the afferent branchial system, so iu the efferent branchial 

 system, the component arteries are arranged in pairs, and the pairs 

 correspond, though the paired condition is less marked in the eflFerent 

 than in the afferent system. 



