452 Dull — Hinge of Pelecypods and its Development. 



In a protective armor like the valves of bivalves, other 

 things being- equal, it will be obviously beneficial if not abso- 

 lutely essential that it should offer as few weak joints or open 

 spaces as possible. Burrowing animals, who themselves serve 

 as a supplementary defence of their burrow, may be able to 

 perpetuate gaping shells and exposed siphons without serious 

 danger from their enemies. Those animals which burrow but 

 slightly or live in material which enemies may also easily pene- 

 trate in their forays, will unquestionably benefit greatly by an 

 accurate and exact closure of the valves. The intrusion of 

 solid bodies can be to some extent guarded against by the 

 action of the cilia or processes of the mantle margin, but such 

 intrusion would be greatly facilitated by any organization of 

 the hinge which would permit an independent rocking motion 

 of the valves with respect to each other. The sudden closing 

 which danger incites leaves no time for clearing ont obstruc- 

 tions and the gap is especially liable to the incursion of gravel, 

 etc., in species which live with the plane of junction of the 

 valves in a vertical direction. In certain brachiopods such as 

 Glottidia and Discina such a semi-rotary motion of the valves 

 exists, but is less dangerous to them since the plane of junction 

 with them appears to be generally horizontal. 



To avoid these dangers and to guide the motion of the valves 

 in closing, and to prevent their sliding upon one another after 

 closing, Nature, through natural selection and physical 

 stresses, has developed these cardinal processes which are 

 known as teeth. 



Attention has already been called to the fact that there can 

 be but three fundamental types of hinge, which may be called 

 the anodont, prionodont and orthodont, the latter term being 

 used to indicate the forms in which the cardinal margin has 

 become longitudinally plicate. Actually the pure orthodont 

 type hardly exists ; in nearly all forms traces of the prionodont 

 characters are mingled with it. For those forms in which the 

 archaic anodontism still persists as the characteristic of chief 

 importance, though frequently modified by special mechanical 

 contrivances which to a certain extent mask the type, I have 

 proposed the term Anomalodesmacea. The fossette, cuilleron 

 or spoon-shaped process for the cartilage is a separate develop- 

 ment serving a special purpose ; though influencing the teeth, 

 if any exist, in its vicinity, it must not be confounded with 

 them. The weakness of the anodont type has left an opening 

 for the specialization and perfection of this process which, to a 

 considerable extent in this group, assumes the functions which 

 in groups without a cartilage are the special office of the 

 teeth. 



For those forms in which transverse plication of the hinge is 

 the chief characteristic, though rarely wholly exclusive of 



