104 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



Example. 



PI. LXXX. Fig. 1 and 2. 



Cypricardia oblonga, Sowerby, Genera of Shells, No. 20. Enc. Meth., 



pi. 234. f. 2. 

 Chama oblonga, Linnreus. 

 Chama Guinaica, Chemnitz. 

 Cardita carinata, Bruguiere. 

 Cypricardia Guinaica, Lamarck. 

 Cardita Guinaica, De Blainville (subgenus Cypricardia). 



Family 10. ARCACEA. 



Testa plerumque sequivalvis ; cardine dentibus parvis numerosis in- 

 structo, in seriem vel rectam vel arcuatam dispositis. 



The generic type of this family may be said to exhibit a most distinct 

 and well-defined assemblage of characters ; the teeth of the shell are 

 small and numerous, set in a straight or curved line, and the ligament 

 is external. Yet we are compelled to admit with the Arcacea, because 

 of the similarity of its animal, a genus in which a direct change takes 

 place in the position of the ligament, it being internal, as in the shell of 

 the Mactracea. Now, when we meet with such an anomalous union of 

 characters as are exhibited in the shell of Nucula, the difficulty becomes 

 embarrassing, because the arrangement of the ligament forms no incon- 

 siderable feature in determining families ; we must, however, be guided 

 by those affinities which are esteemed of most importance in the organ- 

 ization of the animal. For this reason we are led to follow Lamarck and 

 Gray, by retaining in this family a genus which some eminent concholo- 

 gists have entirely removed. 



The shell of the Arcacea is generally equivalve, and the hinge consists 

 of a row of numerous small teeth set either in a straight or curved line 



