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 hgament 
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- 
trmes 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 im our 
days, he would have seen cause to change his opinion, and that 
