224 



And it would seem as if the external hyoidean gill, in so far as the 

 general course and arrangement of the arterial vessels is concerned, 

 might be either a displaced internal gill, as Hyrtl assumed, or a 

 specially developed external gill, as Budgett concludes. If it be an 

 external gill, the total absence, even in Budgett's 30 mm specimen, 

 of any apparent remnants of the vessels of the internal gill would 

 seem to need explanation ; and such an explanation may perhaps be 

 given in the assumption that the thymus is developed largely from the 

 hyoidean gill. 



Pollard and Budgett both compare the dorsal arterial system 

 of Polypterus with that of Chlamydoselachus, and speak of the arran- 

 gement in both these fishes as being exceedingly primitive. This is, 

 however, certainly due to a misconception, probably by both these 

 authors, of the homologies of the vessels both in Polypterus and in 

 Chlamydoselachus. 



This is evident, in so far as Pollard is concerned, from his 

 statement (1892, p. 418) that „The existence of the precardiac portion 

 (of the dorsal aorta) was mentioned by Müller and it is of impor- 

 tance for it is exceedingly primitive. It occurs (Aters) in Chlamy- 

 doselachus and several sharks and is one more indication of the genetic 

 connection between them and Polypterus." What Ayers (1889) defines 

 as the precardiac portion of the dorsal aorta of Chlamydoselachus is 

 all that part of the vessel that lies anterior to ,,the junction of the 

 fifth pair of aortic roots with the sub-chordal vessel" ; and he divides 

 this section of the aorta into cranial, vertebral and branchial portions. 

 What Pollard defines as the precardiac portion of the aorta of 

 Polypterus is that part of the artery that lies anterior to the point 

 of junction of the eff'erent arteries of the second branchial or systematic 

 arches. And it is quite evident, from the context, that Pollard is 

 comparing this portion of the aorta of Polypterus not with the entire 

 precardiac artery of Chlamydoselachus as defined by Aters, but with 

 its so-called cranial portion only. But this so-called cranial portion 

 of the aorta of Chlamydoselachus is not a part of the aorta at all, as 

 fully explained in my work now in press (Allis, 2), and it is conse- 

 quently in no way the homologue of the so-called precardiac portion 

 of the aorta of Polypterus notwithstanding that it resembles it in that 

 both are enclosed in a canal in the base of the occipital portion of 

 the skull. W'hether these aortal canals in the two fishes are in any 

 way homologous, or not, it is not possible to definitely say, but they 

 would certainly seem not to be. The canal in Chlamydoselachus is 

 said to begin immediately in front of the point where the median 



