BIVALVIA. 



INTRODUCTION. 



The Bivalvia constitute the third portion of the still greater division of the animal 

 kingdom called Mollusca,from the word mollis, a term that might byhypercritics be considered 

 objectionable, conveying as it does an erroneous impression, the exterior covering of these 

 animals being in most instances particularly hard, more so than the internal skeleton of the 

 Vertebrata. Bivalves are acephalous, or animals without a head, but they are furnished 

 with a (generally) large and powerful mass of flexible material, called a foot in consequence 

 of its being the principal organ of locomotion ; this is of importance to the Pataontologist, 

 inasmuch as the opening or sinuated edge of the valves at the margin is modified by the mag- 

 nitude of this foot. It is supplied with circular fibres for extrusion, and longitudinal fibres 

 for retraction. Although some of these animals are capable of changing their places 



' In the second volume of the 'Crag Mollusca ' the word "Bivalvia" is employed for those animals 

 that are 'without a head, and are enclosed within two valves or pieces of calcareous covering, and it is re- 

 peated here for the same group. This name appears to me to possess the greatest claim on account of its 

 priority of date, and it is most expressive in its application. An objection has been made to it, that it does 

 not include the Brachiopoda, which are also enclosed within two valves, but the same want of isolation may 

 be urged against the rival claims of Acephala, Conchifera, and Lamellibranchiata. The meaning of each 

 of these names is not restricted to the characters of the animals they are intended to represent, and the 

 advantages they offer are not, in my opinion, sufficient to entitle either to supersede the earlier name of 

 Bivalvia. The Committee of the British Association, in their ' Rules and Regulations for Nomenclature ' 

 proposed in the year 1842, and recommended that "The law of priority should be admitted as the only 

 effectual and just one," but that it should not extend to authors anterior to Linnseus (ed. 12th, 1/67). 

 The adoption by me of the above name is in compliance with those recommendations. 



