NOTES ON JAPANESE BITXJNOIDS. 19 



the fusion of these elements occurs in //. bürgen as in Myxine. As 

 faros my observations go, furthermore, there is considerable con- 

 stancy in the degree of bifurcation of the afferent aorta and in 

 the degree to which the tongue muscle extends hindward, both 

 characters measured with reference to the position of the branchial 

 sacs. The number of the mucous pores, whether " pectoral " or 

 "abdominal," on the other hand, appears to me of minor value : not 

 only do they vary in number palpably, but the anteriormost as well 

 as the posteriormost are very difficult to count, so that in a given 

 specimen, one cannot I believe, conclude what the number is without 

 allowing for a possible error of two or more on either side, a discrepancy 

 which in many cases is sufficient to invalidate the usefulness of this 

 specific test. 



The character of the eg(^ on the other hand, is found of no little 

 importance in determining species : for each species presents well 

 marked differences in the number, size and shape of the eggs, and in 

 the number and distribution of the anchor filaments at the poles. 



The above considerations have, as far as possible, been taken into 

 account in the case of two genera of Hyperotretes in the preparation 

 of the following synopsis. 



(Class) MARSIPOBRANCHII. 

 (Sab class) HYPEBOTBETA. 



(Order) MYXINIA. 



(Family) MYXINIDAE. 



(Genus I) HOIYIEA Fleming, 1822. Phil. Zool, II, .374. (=Bdel- 



lostoma, Müller, 1834). 



Hyparotretes with branchial apertures, six to fourteen in number, 

 lateral of branchial sacs. Ectal efferent branchial ducts of approx- 



