x INTRODUCTION. 
tical, which, on careful comparison, are found not to be so. And 
this is very likely to occur, where some one very remarkable pecu- 
liarity exists. Thus, a Lutraria from Lower California (L. undulata), 
has the thin, milk-white, concentrically undulated valves, so similar to 
those which characterize a shell from the coast of Carolina (L. canali- 
culata), that no one observing them separately, would hesitate to pro- 
nounce them the same; but place the two side by side, and it will be 
seen that in one the beaks are near the posterior, and in the other 
near the anterior end of the shell. Equally striking resemblances and 
differences will be found when we compare Mactra nasuta and M. 
Brasiliana, Lutraria ventricosa and L. carinata, the former of which 
are found in the Gulf of California, and their analogues in the Gulf of 
Mexico. So too we find on the catalogues Cytherea chione and Natica 
maroccana, Mediterranean shells, set down as found also in the Gulf 
of California; but a direct comparison shows them to be quite different 
in form and coloration, and well entitled to the distinctive appella- 
tions of Cytherea biradiata and Natica Chemnitzii. Triton nodosum, 
of the West Indies, has also been regarded as identical with a Sand- 
wich Island species (7. elongatum). We need not multiply examples of 
this kind. But if such confusion has arisen among strongly-marked 
species, how much more liable is it to occur where specific differences 
are slight. In many genera, as in Physa and Succinea, the form, 
surface, and colouring are so uniform throughout, that undoubted 
species are distinguished by only the slightest differences. Indeed, 
there are even some genera, like Helix and Nanina, Patella and 
Lottia, which cannot be distinguished but by an examination of the 
animal. When, therefore, we have before us shells from widely 
diverse regions, apparently identical, they should be subjected to the 
most careful scrutiny for structural differences. If no obvious ones 
are detected, we may not consider the question as settled, unless the 
animals have been compared; and we may go even further, and 
require that their internal structure, as well as external features, should 
be examined. The number of instances where this apparent ubiquity 
obtains is fast diminishing, as in the cases already mentioned, in those 
