214 



CASPIAN SEA. 



Norfolk, more particularly in the fluvio-marine 

 portions. The peculiarity of the shell consists in 

 its being thin and smooth ; the hinge is nearly 

 edentulous; rudiments of a single tooth in each 

 valve may be detected in young shells which finally 

 disappear. The animal is a true Cockle, but the 

 shell is wanting in all the usual characteristics of 

 the genus. 



The Caspian Sea, with its very limited Molluscous 

 fauna, makes us acquainted with another series 

 of aberrant forms of Cardium in which the hinge 

 undergoes great modifications, and which are ac- 

 companied by changes in the form and other 

 characters, to such an extent that they have been 

 referred even to other genera, such as Corbula and 

 Pholadomya. 



To these forms, but allied to them by correspond- 

 ing modifications, may be added as many as twenty 

 others, to which M. Deshayes has given distinct 

 specific names, and which are found in the deposits of 

 the older extensive Caspian. The hinge structure of 

 this group taken altogether presents every conceiv- 

 able deviation from the normal formula of the genus 

 Cardium : the lateral teeth are suppressed, either 

 one or both, and the central ones preserved ; and 

 the reverse take place. Often one tooth is alone 

 preserved, and this is sometimes the anterior, and 

 sometimes the other. This single tooth at times 

 acquires a great development, and is accompanied 

 by a great distortion of the shell on that side \ in- 



