INTRODUCTION. ix 
Some of the disadvantages of taking such a great range of formations are obvious. 
For example, it is almost impossible to collect personally from such an extensive 
series of beds, or to become sufficiently familiar with their stratigraphical details 
and foreign equivalents. The first of these objections is to a large extent removed 
by the magnificent collections which have been made by many enthusiastic and 
careful workers in the Cretaceous rocks. Some of these collections are still in 
private hands; many are now preserved in private museums; but all have been 
placed freely at my disposal. 
Although works dealing with the fossils of limited horizons are often of great 
service to stratigraphical geologists and collectors, yet, from a paleontological 
standpoint, such works are apt to be somewhat unsatisfactory, since, owing to the 
want of sufficient material for comparison from other horizons, the importance of 
slight differences is hable to be overrated, and a proper idea of the variability of 
the species can scarcely be obtained. Further, some of the differences between 
forms from different beds and successive horizons are found to be due merely to 
dissimilar preservation, or are connected, just as is the case at the present day, 
with the varying conditions under which the forms lived. Moreover, the knowledge 
of a genus obtained from the study of a number of species from various horizons 
is obviously much more thorough than when only a few forms from one horizon 
are being considered. 
From a biological standpomt the most satisfactory method would be to study a 
small group, such as a genus or family, and trace it through all formations from its 
earhest appearance to the present day or to the period of its extinction; and 
further, not to limit oneself, as is usually done, to a single country, but to study 
the representatives found in all parts of the world. The difficulties of obtaining 
specimens and of undertaking such extensive travel as that method of work would 
involve are very great; but quite as great, in the case of Lamellibranchs, is the 
difficulty of becoming familiar with the enormous literature which exists on 
this group of molluscs from every geological system. Consequently this method 
can scarcely be attempted until monographs on the Lamellibranchs found in all the 
geological systems of most countries have appeared. Moreover, such monographs 
are urgently needed in stratigraphical investigations. So that, great as is the 
labour involved in the preparation of a monograph on the Lamellibranchs of any 
geological system, it can scarcely be regarded as more than a necessary preliminary 
to the work which will be carried out in the future on many interesting problems 
in phylogeny and stratigraphy. 
Amongst the collections which have been studied in the course of this work 
are those in the British Museum, the Museum of Practical Geology, and the 
Geological Society of London; the Museums of Bath, Bristol, Brighton, Cambridge, 
Exeter, Norwich, and York. In all cases help has been freely given by those 
b 
