ACEPHALA LAMELLIBRANCHIATA. 29 



ternal, or half within and half without, its modus operandi in 

 either case is as I have described; and the ligament, as far 

 as its power extends, acts precisely the same. The ligament 

 is always exterior in reference to the cartilage, often covers it, 

 and is fixed to the external margins of the shell, whilst the 

 cartilage rests on the internal marginal surfaces and callosi- 

 ties that act as a fulcrum, which in many species is aided by 

 an apophysis, pit, or excavated internal process. The teeth, 

 with which a large portion of the bivalves are furnished, are 

 strictly of a calcareous nature ; and I am not aware that they 

 have other uses than literally to act as a hinge, and to assist 

 the ligament and cartilage in strengthening the closure of the 

 valves and maintaining their proper position. 



In some genera where there are no teeth, an accessorial 

 ossicle clasps the two valves as an equivalent; and in those 

 that are entirely mute, the power of the adductor muscles is 

 invariably increased. I have stated, in the preceding paper, 

 that the teeth, ligaments, cartilages, and cicatrices of the 

 muscles, afford, from their variableness and instability, no 

 efficient assistance in forming a continuous natural order; 

 therefore these appendages of the shell will receive no further 

 consideration in any remarks I may make on the natural 

 arrangement of the families of the Acephala ; they may, how- 

 ever, assist in specific distinction, and as accessories and make- 

 weights in balanced investigations. 



I will now make a few observations on the lacunose doc- 

 trines that have lately been promulgated by the French and 

 Belgian naturalists, which we believe have their origin in M. 

 Cuvier's celebrated Aplysia case, in which it is stated that the 

 vena cava or great branchial vein is perforated so as to permit 

 its contents to mingle with the fluids of the abdominal cavity. 

 M. Cuvier considered this structure so unusual, though he 

 communicated it to his friends, that he hesitated to make it 

 public for many years, until he had by every possible test 

 satisfied himself of this almost unnatural phenomenon, which 

 he declared to be the most extraordinary he had ever met 

 with. I think, if that illustrious naturalist had lived in our 

 days, he would have seen cause to change his opinion, and that 



