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 palasontological 

 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 liable 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 standpoint 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 

 earliest 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 Lamellibraiichs, 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 Lamellibraiichs 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 Lamellibraiichs 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 



