INTRODUCTION. 3 



Occasionally the original type is absent from some of these collections, and at 

 times its place is taken by one or more specimens from the same locality whence 

 the original specimen was obtained. A certain number have, unfortunately, 

 altogether disappeared ; and it will be necessary, owing to absence of any 

 accurate and detailed diagnosis, to drop certain names of long standing, since it is 

 impossible now to obtain the slightest indication of what the original shell was 

 like. An example of such a case is the Modiola (jranulosa of Phillips, the original 

 of which is stated to have been drawn from a specimen in the Museum of the 

 Yorkshire Philosophical Society. But there is no specimen there that in any way 

 can be referred to the drawing, and the whole of the description is contained in 

 the five words " very elongated, depressed, surface granulose," which convey no 

 meaning of any diagnostic value. 



There is a very great initial difficulty in determining the proper place in 

 general classification which the Palaeozoic genera of the Lamellibranchiata should 

 occupy. In many cases no recent representatives- seem to exist ; and, even with 

 full details as to hinge-structure, it is a matter of great uncertainty as to which 

 family a genus should be referred. Several of the Carboniferous genera seem to 

 fall into natural groups ; for example, Ganltomorpha, de Koninck ; IsocuUa, 

 M'Coy ; Broekia, de Koninck ; Pachydomus, Morris ; Edmondia, de Koninck ; and 

 Scaldia, de Ryckholt, have much in common, but can be distinguished by well- 

 marked characters ; but the real position of this family is so uncertain that de 

 Koninck, in spite of the absence of a sinuated pallial line in all members of the 

 group, places the six genera under the Anatinidse ; while M'Coy refers Edmondia 

 to the M'ljtilidae, and Cardiomorpha to a new family placed directly after the 

 Mytilidx. More light will be thrown on these still obscure points only when a 

 careful comparison can be made between the Palgeozoic and Mesozoic shells, to 

 permit of which it will be necessary to obtain the hinge-plate of each genus. 



It is impossible, in the present state of our knowledge, to attempt to arrange 

 the various genera under their proper families ; but the writer hopes co be able to 

 do so at the end of his work, when the tangled web of synonymy shall have been 

 unravelled, and some definite information has been obtained of the genera of 

 the Lamellibranchiata which existed in Carboniferous times. Owing to the 

 want of material and other causes, the chief of which is the very excessive 

 number of species that have been figured and described by various authors, it has 

 been necessary to leave the study of the Carboniferous representatives of the 

 families Pectinidse and AvicuUdse for a future time. This work, therefore, 

 commences with genera of the family Mijtilidse, which existed in Carboniferous 

 times. An attempt will be made to proceed systematically from this point, 

 though, owing to the present uncertainty as to the true family position of certain 

 genera, some rearrangement may be found to be necessitated in the final resmne. 



