584 The American Naturalist. [July, 
eight pairs in existing forms.” The deviation from this 
almost universal rule led me to propose the generic differ- 
entiation of “ Bdellostomids with an increased number of 
branchise" from those “ with typically 7 (sometimes 6).” Be 
it recalled also that the former have * the base of the tongue 
between the seventh or eighth pairs of gills," while the latter 
have *the base of the tongue between the anterior pair of 
gills.” The genera thus defined were named by me Polisto- 
trema and Heptatrema (Proc. U. S. Nat. Mus., 1882, pp. 518, 520). 
These have been accepted by Jordan, Gilbert, the Eigenmanns, 
and others, and probably will continue to be. Dr. Ayers, 
however, has urged that “these accounts all refer to the 
varieties of what I shall call Bdellostoma dombeyi, adopting 
Müler's genus on account of the inapplicability of Lacépéde's 
Gastrobranchus, and of theinappropriateness of Cuvier's Hepta- 
tremes, which could only be used for the seven-gilled form or 
variety " (p. 155). 
Gastrobranchus was a generic name formed for Myxine alone 
and of course could not be perverted to a Bdellostomid. Hep- 
tatrema can be used for the group to which it was applied with 
perfect propriety, even though the species deviate in having 
often 6 branchial apertures on one or both sides. A cor- 
responding latitude of usage is so generally recognized by 
modern zoologists, that a defense of such procedure is unnec- 
essary. Even if such an extreme view prevailed, however, 
there is the name Homea of Fleming available, and this was 
proposed many years before Bdellostoma. 
There are several other questions that deserve attention, 
but I resist the temptation to consider them now. 
?- The relation of the tongue muscle to the gills is of interest, and here again 
we find great variability. Müller found it to lie entirely in front of the gills in 
the 6 and 7 gilled forms from the Cape of Good Hope, and this condition obtains 
in Myxine so far as known. In Bdellostoma with 10 or 11 gills, the base of this 
muscle may lie between the 6th and 8th pair of gills according to Putnam. In 
the 12 and 13 gilled forms, I have found it between the 5th, or at most, the 6th 
pairs of gill-sacks.” (Ayres, p. cit., p. 139, 140). No observational basis has fill- 
ed the great gap between the “front of the gills” and the interspace between 
the 5th pair!” 
