No. 5. — The Morphology of the Carotids, based on a Study of the 

 Blood-vessels of Chlamydoselachus anguineus, Garman. By H. 



A.YERS. 



Chlamydoselachus * holds undisputed claim to being the most 

 lowly organized Elasmobranch yet discovered, and it was to have been 

 expected that primitive conditions of organization would be retained 

 to a greater extent than in any other known member of the group, the 

 vascular system not excepted. 



Almost nothing is known of the vascular system of the Notidanidas, 

 but it may be inferred from a comparison of their other structures with 

 the corresponding organs in Chlamydoselachus that their vascular sys- 

 tem will not be found to retain all the primitive characters present in 

 Chlamydoselachus. This primitiveness of structure is expressed, 1st, 

 in the retention of a large number of aortic arches ; 2dly, in the pres- 

 ence of the complete dorsal aorta, of which the precardiac portion among 

 the remaining vertebrates is almost without exception either extensively 

 or completely atrophied ; 3dly, in the extensive venous spaces, always 

 simple in character, developed in the course of the large venous trunks. 



While at the Banyuls-sur-Mer zoological station in the spring of 1885 

 I reached the conclusions, (a) that the vascular system of existing 

 sharks had been extensively abbreviated in the course of descent in con- 

 nection with the development of the head ; (b) that formerly there must 

 have been a much larger number of aortic arches than we now find in 

 any member of the Elasmobranch group ; and (c) that with the loss of 

 the aortic arches the dorsal aorta of the branchial region had disap- 

 peared either entirely or in part. As examples of the latter condition 



* The two papers containing the original descriptions of the systematic position 

 and anatomical characters of Chlamydoselachus anguineus from the type in the 

 Museum of Comparative Zoology at Cambridge, Mass., include all that is known 

 of the creature : — 



1 Garman, Samuel. New Sharks, Chlamydoselachus anguineus, etc. Bull. 

 Essex Institute, Vol. XVI. p. 1. 



2 The same, 1884. Chlamydoselachus anguineus, Garman. A Living Species 

 of Cladodont Shark. Bull. Mus. Comp. Zool., Vol. XII. No. 1. Cambridge, Mass., 

 1885. 



VOL. XVII. — NO. 5. 



