208 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 
NOTE ON THE GENERA OF PETROMYZONTID#. 
By DAVID S. JORDAN and CHARLES H. GILBERT. 
In the Proceedings of the National Museum for 1882, p. 521, is a 
review of the genera of Lampreys, by Dr. Gill. In the analysis of the 
genera several characters not hitherto recognized have been noticed, 
and the paper is a substantial addition to our knowledge of these ani- 
mals. The characters drawn from the dentition of the supraoral lamina 
are, however, unreliable for the distinction of genera. Thus in Petro- 
myzon and Ammocetes this lamina is said to be bicuspid, while in Tehthy- 
omyzon and Entosphenus it is described as tricuspid. In Petromyzon 
and Entosphenus, so far as we have noticed, the description given by 
Professor Gill fully applies. In Jchthyomyzon the supraoral lamina is 
bicuspid in the common species, I. argenteus, but tricuspid in J. casta- 
neus and I. hirudo, species not generically distinct from I. argenteus, and 
whose specific validity, indeed, may be questioned. Ammocetes usually 
has a broad supraoral lamina, with a tooth ateach end, but very often, 
at least in our common species A. niger, there is also a median cusp, 
as in Entosphenus. We have no doubt that Ammocetes (as understood 
by Dr. Gill) should be generically separated from Petromyzon. The 
characters of Jchthyomyzon and of Entosphenus are, however, of less 
importance. The teeth of Ichthyomyzon are all essentially as in Petro- 
myzon, but proportionately smaller, weaker, and with less developed 
points and serre. The same is nearly true of the teeth of Ammocetes 
in comparison with those of Entosphenus. Petromyzon and Entosphenus 
are large marine species, ascending fresh waters only to spawn (or occa- 
sionally land-locked), while Ichthyomyzon and Ammocetes are their re- 
spective fresh-water representatives, smaller, weaker, and less special- 
ized. Convenienceis probably best served by recognizing all four groups 
as genera. 
INDIANA UNIVERSITY, April 8, 1883. 
aR ORE LEON, OF A NEW MURZENOID EEL (SIDERA CHLEVASTES) 
FROM THE GALAPAGOS ISLANDS, 
By DAVID S. JORDAN and CHARLES H. GILBERT. 
Sidera chlevastes sp. nov. (20385). 
Body little elongate, strongly compressed. Head rather small, some- 
what compressed. Teeth rather strong, most of them slender and sharp; 
their edges entire. Teeth of lower jaw uniserial, directed strongly 
backwards, close set, slightly increasing in size posteriorly; about 14 
on each side. 
Teeth of upperjaw biserial, for most of its length; the teeth of the 
