PREFACE. 
IX 
plates were lithographed, from i to lxxx, for illustrating the material then on 
hand, and the additional plates can only come in after the latter number. This 
want of conformity, however, affects only the plates, the text presenting the 
genera and species in consecutive and systematic order, with cross-references 
to the newly introduced plates. 
The subdivision among those forms which have usually been referred to 
Aviculopecten was found to be imperatively necessary for any strict classifica¬ 
tion, and the other subdivisions aniong generic forms which have indiscrimi¬ 
nately been referred to Avicula, Pterinea, Pteronites, etc., have been deemed 
equally important. In these subdivisions, while the essential internal characters 
have been regarded as of primary importance, the author, without violating 
this rule, has endeavored to make such an arrangement of the species that the 
student may determine the generic relations from the general form and exterior 
markings alone. Since the fossil Lamellibranchiata are commonly found in an 
imperfect condition, with the interior surface remaining attached to the matrix, 
and only in rare examples, or in very favorable conditions, revealing the interior 
structure, it becomes important to have some means of identification other than 
that furnished by the hinge characters which are so rarely accessible. However, 
notwithstanding the general arrangement according to external features, the 
hinge and the interior structure have by no means been undervalued or neglected, 
as is shown by numerous illustrations of these parts, which it is believed give 
an amount of information not before published in any work upon this class of 
fossils. 
The synopsis of genera here presented is given without comment or com¬ 
parison. In the succeeding volume the author proposes to give a resume of all 
the genera described, and in the same connection a comparison with genera 
described in other publications with some notice of the bibliography of this 
class of fossils, so far as relates to the palaeozoic forms. In that notice the 
reasons for the generic subdivisions proposed in the present volume will be 
given in full. 
In the final revision of the genera and species and in the preparation of the 
B 
