BIVALVIA’) 

INTRODUCTION. 
Tp Brvatvta constitute the third portion of the still greater division of the animal 
kingdom called Mollusca, from the word mod/is, 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 Paleeontologist, 
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 
1 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 caleareous 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 itis 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 culy 
effectual and just one,” but that it should not extend to authors anterior to Linnzus (ed. 12th, 1767). 
The adoption by me of the above name is in compliance with those recommendations. 
