42 W. Stimpson on the so-called Melanians of N. America. 
that Iam convinced that they cannot be separated genericall 
Se other than conchological grounds. The same is true wit 
rd to some other genera of ths family, in iihoh even the 
neric a eee se But let it not be understood from these re- 
marks that I deny the convenience and expediency of keeping 
separate as genera these groups founded on the characters of the 
shell. As long as our nomenclature is binomial and not trino- 
mial, every well-marked group of species should be kept distinct 
as a genus, however trivial in importance the characters upon 
which it is based may seem to be; and this is all the more ne- 
cessary in families containing a very large number of forms. A 
genus is simply the ultimate subdivision of species considered in 
respect to their relations to each other, and the elimination of 
the term “subgenus” from our categories seems to be require 
by the very nature of our binomial nomenclature. 
The following brief account of the structural characters of 
Senay Credobrandliite Gas pte as is Bn ed by the 
diverse character of the external ahs organ 
General Characters.— ‘he head is ‘phovided with a 
rather large and somewhat elon ated rostrum, tapering very 
mgt _and projecting normally considerably beyond the anterior — 
n of the foot. This rostrum or snout possesses consider- 
ible mobility and has a strong resemblance in physiognomy 
to that of a hog, of which one cannot fail to be reminded while 
watching a crowd of these Melanians feeding at the river margin. 
It is somewhat contractile, and is wrinkled transversely upon 
the upper surface, except when protruded to its full extent, when 
the oe isappear. In the living animal it is but little de- 
t it becomes much flattened in spirits. It is termina-— 
ted by ity oval disk, which is emarginated at the middle in front — 
and behind; the mouth is indicated by a longitudinal slit zs the 
centre, extending nearly to the posterior margin. The ceal 
| ee is very little retractile, and its retraction is not ssschepatal 
by an oe eo of the extremity of the rostrum. The jaws, 
according to Troschel,*® con very numerous little scales, 
which Bed a polygoral, and for pe most part a hexagonal, form. 
teeth on the lingual ribbon are arranged in seven rows, 
@ dy: 8), and their characteristics, according to the author above 
quoted, (whose observations on this point I have in part 
