147 



mistaken for, any other genus. Only six species are de- 

 scribed by him, though many more are known to collec- 

 tors in England. The principal observation Lamarck 

 makes on this singular and well-known shell, is, that it 

 seems more like an univalve than a multivalve, and 

 should only be considered an elongated shell, with the 

 testaceous portions of it transversely broken by nature, 

 at its first formation, into a number of distinct moveable 

 parts, in order to facilitate the animal's motion : these 

 valves or testaceous parts, which are generally eight in 

 number, and as Lamarck asserts sometimes only seven, 

 and even six, are inserted at their lateral extremities into 

 a tough ligament, which surrounds the outer margin, 

 and firmly unites them together in their proper position. 

 This ligament is sometimes fleshy, coriaceous, smooth,or 

 wrinkled, covered with small scales, or beset with tufts 

 of hair; and in one species, C.spinosus, it has numerous 

 tubular, curved, long, thin, black spines. A tough inter- 

 nal membrane connects the under part of the valves or 

 portions, allowing the animal to contract the shell in 

 a globular form, or fully to expand it; in which posi- 

 tion the edges of the valves slightly overlap each other. 

 When these portions are all united, they form an ellip- 

 tical shell, the centre part of which is more elevated 

 than the rest; and each succeeding valve of less con- 

 l c 2 



