238 REPORT ON ALBATROSS MOLLUSCA DALL. 



The resistance of the ligament to compression prevents any straining of 

 the adductors by a too wide opening of the valves. The same resistance 

 in the cartilage prevents the ventral margins from crashing each other 

 by sudden and violent contractions of the adductors when the animal 

 is alarmed and closes its valves. The uymphaj, or processes to which 

 the ligament is attached, and the fosse tte or socket of the cartilage 

 have been strengthened aud regulated by the development of various 

 buttresses aud other devices, varying in different groups. The carti- 

 lage in turn has its rigidity and strength increased in many species by 

 the special development of shell substance known as the ossiculum. 



To return to the development of the cardinal margin. The asymme- 

 try of the shell and ligament, relative to a vertical transverse plane 

 passing through the umboues, would be promoted not only by the nat- 

 ural discrepancies between the anterior and posterior halves of the body, 

 but by the mechanical effect of the projecting uinbones. Where a shell 

 opens laterally in the strict sense of the word, unless the beaks are very 

 inconspicuous, or are separated by a wide projection of the cardinal 

 border (as in Area now), they will strike against and wear out one an- 

 other. This abnormal or accidental result is very constantly observable 

 in many Anatinidce, such as our own Thracia conradi. But it must be 

 a source of weakness and danger to the animal. If the ligament is 

 shifted posteriorly, the valves must open more obliquely, with the result 

 that this dangerous friction will be avoided in most cases. 



In a protective armor like the valves of bivalves, other things being 

 equal, it will be obviously beneficial, if not absolutely essential, that it 

 should offer as few weak joints or open spaces as possible. Burrowing 

 animals, who serve themselves of their burrow as a supplementary 

 defense, may be able to perpetuate gaping shells and exposed siphons 

 without serious danger from their enemies. Those animals which bur- 

 row 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 auy 

 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 out obstructions, aud 

 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 direc- 

 tion. In certain brachiopods, such as Glottidia aud Discina, such a semi 

 rotary motion of the valves exists, but is less dangerous to them since 

 the plane of juncture with them appears to be generally horizontal. 



To avoid these dangers and to guide the motion of the valves in clos- 

 ing and to preveut their sliding upon one another after closing, nature 

 through natural selection and physical stresses has developed these 

 cardinal processes which are known as teeth. 



