SEELEY — ( 
■OS FOSSIL BIVALVE SUELLS. 
45 
analysis of rocks into their several zones, it is on the lower forms we 
rely in synthetical arrauwements. The Conchifera readily divide 
rocks into Palaeozoic and Neozoic, — no form with a pallial sinus being 
known below the Lower Secondary strata ; while, from the appear- 
ance in them of numerous new genera, no class of animals better 
marks the recognized systems into which fossiliferous rocks are 
grouped. It is indispensable both to the geologist and biologist to 
be familiar with the genera, and it is the object of this paper to 
render the principles on which they are identified more easy and 
exact. 
That genera are practically realities every student knows well ; 
as such the geologist and zoologist have to do with them. And it 
has elsewhere* been shown that between hosts of groups interme- 
diate forms can no more be found, than can intermediate wood fill 
the space between the forked branches of a bush. Accepting the 
fact, the question arises, — How may genera be known ? The dis- 
i tinctive characters depend on the definition of a genus adopted. If 
' it is merely a number of nearly-related species, then the descriptive 
characters will include those in the whole of the forms ; but if it 
exists independently of the species, being fundamental to them, then 
the specific characters cannot enter into the description of the genus. 
Practically this latter view, which is adopted in the paper referred to, 
reduces the description to a sixth of the shortest customary length, 
while the distinction becomes clearer by pruning away much that is 
I common to other types. But even these characters are often need- 
lessly redundant, and several common to nearly-allied groups may 
be cancelled. 
Having in this spirit examined an extensive series of Lamelli- 
branch shells in preparing for a course of lectures, given three years 
ago, I found in most cases the hinge-characters among the most im- 
portant residua so gotten ; and shortly after, while determining some 
casts of fossils in which the hinge-teeth were the only generic cliarac- 
ters seen, it occurred to me to write the teeth do A-n in formulsB like 
those used for the teeth of mammalia. Subsequent re-examination 
of specimens showed that in a large majority of cases these hinge- 
formulse were almost the only characters with which the student 
need be troubled ; and bi'lieving that it will not be found altogether 
useless, the following list has been made of the more common and 
remarkable forms, and arranged for facility of reference. 
The teeth sometimes vary much in the same genus, — one Cardium, 
for instance, showing twelve teeth, and another not more than six, 
! though rudiments of the other six may generally be found ; and 
occasionally the hinge is toothless, the teeth becoming obliterated 
with age. The constant teeth are notated in ordinary numerals ; 
* * Researches on the Homologies of the liivalve Molhisca ; and therein of the Law 
of Variation of Forms, and the Nature of Genera,' Part 1, commnnicated to the Cam- 
bridge Philosophical Society, March 17, 186i, and remaining unpublished, i)ending 
completion. The forinula) used in this paper, w^re th^n explained in a uote^ ; , ^ 
