BIVALVIA. 7 



conjunction with the connexus, constitutes the hinge on which the valves open and close. 

 The Bracliiopoda, which have also two valves, differ from Bivalvia in wanting the 

 connecting ligament, and the opening as well as the closing of the valves is in that class 

 effected by muscles ; they differ also in having one valve perforated, through which the 

 byssus passes, and as this opening is enlarged during the growth of the animal, the infant 

 state of that perforated valve is entirely destroyed, but the infant state of the Bivalvia is 

 visible in both valves at all ages in the uinbonal region of the adult shell. 



The adductor muscles of the Dimyaria are generally of the same magnitude, or at least 

 nearly so ; where there is a difference, it is in those animals whose shells are greatly 

 inequilateral, the larger portion then requiring somewhat greater power in closing the 

 valves. The pedal and siphonal regions are easily known in most species, even in the 

 equilateral or orbicular shells, by the position of the connexus when it is only on one side 

 of the umbo, as in that case it is always more or less on the siphonal side, even in those 

 shells where it is wholly internal or cartilaginous ; but in genera, such as Pectiinculus, 

 Limopsis, and the equilateral Area, which have the connexus spread over a large area 

 equally on each side of the umbo, it is by no means so readily determined. The siphonal region 

 may, however, be generally recognised by the impression of the anal adductor being somewhat 

 higher up, permitting the openings for the incoming and outgoing currents to pass beneath 

 it, and the oral adductor is slightly prolonged by the union of the pedal muscle, which 

 in these animals is required to move their large and expansible foot. 



The Bivalve shell has always been described as having an anterior and a posterior 

 extremity, and it would be desirable still to retain that general description, if it were not 

 for the diversity of opinion there always has been, and which still exists, amongst 

 conchologists, as to which should be considered the anterior and which the 

 posterior portion of the animal. It is at times difficult to determine, in descriptions, 

 which is intended by the author as the anterior, and which as the posterior, for these 

 terms are sometimes employed conversely. T believe the animal in its shell will 

 sufficiently denote the anterior and posterior of itself by the position of the mouth 

 and anus, as in all animals where these are separated the first is considered anterior and 

 the last posterior ; but in the Bivalvia it is said by those who advocate the reverse of 

 this, that the indrawing tube of the animal by which nutriment is conveyed to the 

 palpi, and thence to the stomach, is on that part which by some is called posterior, but 

 which ought, on that account (they say), to be called the anterior, and also that this portion, 

 in all boring Bivalves, for example, is always the uppermost or anterior in their retreat. 



I have therefore thought it expedient in this monograph to substitute for these terms 

 an alteration by which, it is hoped, the confusion may be avoided. That which I have called 

 the pedal region ought perhaps, more strictly speaking, to be considered as the buccal 

 region, and the opposite one the anal, but the Palaeontologist is more especially con- 

 cerned with the shell and its interior, which bears the impress of the muscles only, and 

 not of the viscera, or at least very rarely so. I have thought it better to employ those 



