Introduction. 



A critical study of the literature accessible on the family Chamidae, to which 

 I was led by working on the collections of the Svvedish State Museum, has con- 

 vinced me that our knowledge of this group of Lamellibranchs is in need of a radical 

 revision. Even in attempting to identify species belonging to the group in question 

 one meets with difficulties, because the descriptions of the species are verv incom- 

 plete, and only antiquated and deficient monographs are at one's disposal for de- 

 termination. The descriptions are based exclusively on unessential characteristics 

 such as the exteriör shape, the sculpture and the colour of the shell. An arrange- 

 ment of the species described in the monographs into larger groups is a still more 

 difficult task, as so important an organ as the hinge has been entirely left out of 

 consideration by the authors. Even when subjected to special investigations the 

 hinge conditions have been more or less incorrectly apprehended by different au- 

 thors. As to the anatomical organization of the group one tries also in vain to get 

 a detailed conception. Works on this matter deal with only a few organ systems 

 and are, further, almost exclusively descriptive, taking into consideration some single 

 species of the group, without laying stress upon com parison with other forms. It 

 is evidently impossible to obtain an approximate knowledge of the mutnal relations 

 of this group of animals and thereby a basis of their natural system, as every con- 

 clusion as to the phylogenetic relations is seriously vague, supported as it must be 

 by inadequately described and insufficient facts. 



I therefore made it my task to undertake a close investigation of different 

 forms belonging to the recent Chamidae stored in the Swedish State Museum, in 

 order to gain as exact ideas as possible in all the respects named. In publishing 

 the results of this research 1 hope they will attract the attention of zoologists as 

 well as paleontologists in order that a joint work may fill the large gaps and give 

 an ansvver to the many questions which still remain to be solved but which are 

 impossible to clear up by the efforts of a single scientist, before the knowledge of 

 these Lamellibranchs is complete enough to let us judge with some certainty the 

 problems of evolution which are presented in an unusual degree by the study of 

 the family Chamidae and the allied forms. 



