MUSEUM OF COMPARATIVE ZOOLOGY. 197 



The first and second efferent branchials unite to form a trunk that 

 reaches the aorta as the second aortic root. The third and fourth pairs 

 find independent outlets as the third and fourth aortic roots, while the 

 fifth and sixth pairs fuse directly to produce short common trunks, the 

 last or fifth pair of functional aortic roots. 



The distance between the points at which the first and second and 

 the second and third efferent branchials enter the aorta is nearly the 

 same, while the common trunks of the fifth and sixth enter the aorta at 

 a distance behind the fourth twice as great as that between the other 

 pairs. 



Unlike all other gnathostomous vertebrates, Chlamydoselachus has 

 a dorsal aorta (dorsal vessel) running the entire length of the noto- 

 chord, to which it is intimately attached through the greater part of its 

 course. There is, however, no trace of a chondrification of its walls, 

 such as frequently occurs in cartilaginous fishes (e. g. Sturio). For con- 

 venience in describing as well as on morphological grounds, it is desira- 

 ble to designate two sections of the dorsal aorta by the terms precardiac 

 and postcardiac. The former receives the aortic roots and supplies the 

 head with arteries, the latter gives off" the arteries to the trunk and tail. 



The head and all the precardiac portion (see Figs. 1 and 2) of the 

 trunk are supplied with blood by means of the very primitive mns- 

 culo-spinal arteries and branches of functional or rudimentary aortic 

 arches. This fact is of the greatest importance in any discussion of the 

 homologies of the head arteries, or more exactly precardiac arteries, of 

 the higher Vertebrata. These vessels where more or less rudimentary 

 as regards their main trunks have undergone secondary changes, during 

 which the course of the blood currents through them may have been 

 reversed, and they have usually acquired new outlets, or inlets, as the 

 case may be. 



It is convenient to make a further division of the precardiac section 

 into cranial, vertebral, and branchial portions. The precardiac section is 

 marked off posteriorly by the junction of the fifth pair of aortic roots 

 with the sub-chordal vessel. It terminates anteriorly in the pituitary 

 plexus. The postcardiac section extends from the junction of the fifth 

 pair of aortic roots to the tip of the tail. (See Figs. 1 and 2.) With 

 the exception of a slight enlargement in the occipital region, the diam- 

 eter of the aorta is constant between the occiput and the origin of the 

 coeliac artery ; from this point backwards the aorta gradually tapers into 

 the small caudal artery. From the occiput forwards the cranial sec- 

 tion of the precardiac aorta lies imbedded in the cartilage of the basis 



