MUSEUM OF COMPARATIVE ZOOLOGY. 193 



At present I doubt not that Hyrtl found in every case only an extra- 

 cranial vessel so far as he traced it, and that perhaps he did not follow it 

 quite to its end on account of the non-penetration of the injection 

 mass beyond the point of entrance into the cartilage.* While study- 

 inof the same species at the Laboratoire Arago it did not occur to me 

 to seek for the vessel within the substance of the cartilaginous brain 

 case, and I always found the vessel very much as Hyrtl figures it, 

 though usually I was not able to trace it so far anteriorly as it is drawn in 

 his plate. Since my attention has been called to the matter I have dis- 

 sected only one specimen of Scyllium stellare from the Museum alcohol 

 collections, but the histological condition of the tissues did not permit 

 a satisfactory determination of the relations of the vessel. Further 

 study of this form is very desirable. So far as the homology of the two 

 vessels is concerned, there can be no question that they have strictly the 

 same morphological value. 



To aid in understanding the relations of the carotids, and to serve in 

 the comparison of other forms, as well as to give an idea of the funda- 

 mental simplicity of organization of the vascular system, I shall first 

 describe the aortic system of blood-vessels in Chlamydosolachus passing 

 thence to a consideration of the homologies in other vertebrates in so 

 far as lies within the scope of this paper. 



The sinu-aui-icular (see Figure 2) valve is placed in the middle of 

 the transverse axis of the sinus venosus with its Ions? axis at ri<?ht ansles 

 to the axis of the latter. It is slit-like and guarded on either side by 

 two broad semilunar tendinous folds, — the sinu-auricular valves. The 

 remaining auricular wall of the sinus is smooth, muscular in some 

 de,L,'ree, but thin. 



The aperture of the auriculo-ventricular valve is placed to the left, 

 below and in front of the sinu-auricular valve, and pierces the thick ven- 

 tricular wall. Its ventricular end is provided with two cup-shaped folds, 

 the auriculo-ventricular valves. Its auricular end presents a radiate 

 figure formed by the tendinous cords of the muscular plate, which cen- 

 tre here. The dorsal wall of the auricle is smooth without, but rug-ose 

 within. It is scarcely thicker than the wall of the sinus venosus. The 

 ventral wall of this division of the heai't is thick and muscular, and 

 forms a triangular plate which projects beyond the edges of the ventricle, 



* Miiller certainly did not see the vessel in any species studied by him, for he 

 says (/oc. cit. : " Bei den Haien u. Rocheii felilt die vordere umpaare Fortsetzung 

 der Aorta schon ganz, wis bei den Knochenfisciien aber die Haien besitzen nocli 

 einen circulus cephalicus ini Sinne Hyitl's," etc. 



VOL. XVII. — NO. 5. 10 



