198 BXJLLETIN OF THE 



cranii ; sinking into the cartilage just in front of the posterior border of 

 the basi-occipital cartilage, it runs forwards, gradually rising to the 

 inner surface of the cranial floor, remaining equidistant. from the chorda 

 until near the anterior end of the latter, when the aorta dips slightly 

 to make a bold curve upwards into the pituitary prominence within 

 which it gives ofi' two lateral branches which separate from the median 

 vessel only gradually. (See Figure 3.) These three vessels make their 

 way through the cartilage, and by freely anastomosing with one another 

 form a small but sharply defined plexus, crowning the pituitary promi- 

 nence, but separated from the cartilage by several well defined layers of 

 connective tissue, one of which bridges over the pituitary depression, 

 and thus excludes the internal carotids from the cerebral cavity. The 

 plexus lies external to the dura mater. 



As we have seen, the aorta is made up by the confluence of six pairs 

 of efferent branchial arteries, which pour their blood into the aorta 

 through only four aortic roots, and in this condition we recognize the 

 process of reduction, transposition, or utter obliteration at work in get- 

 ting the creature out of a lower into a higher stage of organization. 

 But the four aortic roots which bring blood from the gills are not the 

 only trunks which from their relations to the aorta and the body make 

 good their claim as aortic arches or roots. As Hyrtl has pointed out, 

 the pair of vessels running from the internal carotid trunk to the aorta 

 in all sharks is most surely an aortic arch, and although it has lost its 

 direct connection with a gill, which we have every reason to believe it 

 formerly had, it still retains its connections with a trunk which has 

 resulted from the obliteration of a series of efferent branchial vessels, 

 and through this trunk an indirect connection with the first two func- 

 tional gills of Chlamydoselachus. Besides this pair, there is another 

 whose relations to the dorsal aorta are such as to entitle them to rank 

 as aortic arches. I refer to the two side branches which the dorsal 

 aorta gives off as it approaches the pituitary plexus. To these six aortic 

 arches I would add a pair represented by the anterior portion of the 

 internal carotid arteries, and another pair represented by the efferent 

 vessels of the spiracular pseudobranch, which pour their blood into the 

 dorsal aorta through the ophthalmic artery, internal carotids, carotid 

 plexus, and pituitary plexus, all of which vascular structures anastomose 

 among themselves from without inwards, in the order given. To these 

 still another pair may be added, recognizable in the last pair of efferent 

 branchial arteries (the sixth functional pair), which by means of their 

 fusion with the fifth pair have not been counted. Chlamydoselachus 



