BIVALVIA. 9 
the umbo to the ventral margin, such as Lima, Vulsella, Ostrea, &c., we are much perplexed 
with the application of these terms, as we meet sometimes with a shell whose height will 
exceed the length two or even threefold, and the longest diameter in that case is in 
ordinary parlance called the length, and this is often so employed by those who estimate 
generally the length of the Bivalve from the pedilateral to the siphonilateral margin. The 
same terms ought in our descriptions to be employed for such shells as O. Virginica, as is 
done to those like So/en, &c., where the longest diameter is in an opposite direction. 
The older Tertiary species of England are, I believe, with a very few doubtful exceptions, 
all extinct, but they have a generic representation among existing forms. This, of course, 
must depend upon what is to be accepted as sufficient for a generic distinction. 
A genus is said to consist of a group of species related by community of structure and 
parts comprehending all essential characters. In Bivalves the xwmber and position of the 
hinge-teeth, as also the position of the connexus, whether external or internal, are considered 
by some authors suflicient, when separately estimated, to characterise a genus; while 
others will regard some of these distinctions as unimportant, or, at least, of only specific 
value.’ The definition of a species is equally unsatisfactory; it has been given in the 
following formula :—‘* All specimens or individuals which are so much alike that we may 
reasonably believe them to have descended from a common stock (parent or parents), con- 
stitute a species.” It is, however, found in the practical examination of individuals that 
the line between these groups is generally so ill-defined that we are led to doubt whether 
there is in nature any such distinction as we find it convenient to make in order to suit 
our own purposes. Great authorities say that “no general rule can be laid down for 
determining the distinction of species, as there is no particular class of characters which 
can serve as a criterion. In each case we must be guided by analogy and probability.” 
It thus appears that our determination is mainly dependent upon individual opinion, and 
we have nothing whereby it ce be decided that the conclusion of any author is the correct 
or real one. ‘Those only who have laboured hard to fix the limit of a species out of a 
multitude of specimens, of similar or proximate forms, can tell the uncertainty attending the 
determination of such a distinction, and the unsatisfactory conclusion at which the 
naturalist often arrives; and when the varying opinions of preceding authors are weighed 
1 In order to show the difficulties which exist in characterising such a section, we may take the 
Genus Lucina as an example. This Genus, as admitted by several modern authors, contains shells possessing 
differences which, in other portions of Bivalvia, are considered as being in themselves separately entitled to 
generic distinction. Thus, Lucina comprises: Ist, shells without hinge-teeth ; 2dly, shells with cardinal 
teeth only; Sdly, shells with one lateral tooth; 4thly, shells with two lateral teeth. This genus also 
contains species in which the connexus is both outward and inward: Ist, those in which it is wholly 
external, with a thick and solid shell, such as LZ. columéella ; 2dly, those in which it is amphidesmous, with 
a thin and almost diaphanous shell, such as LZ. flewuosa ; 3dly, those in which it is situated internally, 
opening the shell by expansion, as in L. lactea; besides these differences, some species are perfectly plain 
and free from sculpture, while others are beautifully ornamented, and the animal also partakes of the 
varying characters of the shell. 
2 
