1^8 
so long known as fossils, under the name hypocephaloides, a represen- 
tation of the calcedonic cast, from Tilsbury, is given PL XII. Fig. 12. 
One circumstance respecting the shells of this genus demands parti- 
cular mention. Lamarck has figured, in the recent shell T. margari- 
tacea two marks of attachment, one at the bottom of each tooth of the 
hinge ; but I have not been able, in any of the fossil shells which I 
possess, to discover more than one mark of attachment, which is on 
one side ; a considerable cavity behind the hinge-tooth, on the other 
side, at a, Fig. 1 and 2, seems to be well calculated to receive a part of 
the animal, which might be attached to the posterior part of this cavity. 
Cl I. Tridacna. A subtransverse inequilateral shell : the hinge 
formed of two compressed and entering teeth ; the crescent, or 
posterior depression, gaping. 
The shell, to which this genus is appropriated, is, Tridacna gigas, 
Brug. or Chama gigas, Linn. 
Fossil shells of this genus are, I believe, very rare. They have been 
said to exist in the neighbourhood of Verona, and of a very consider- 
able size ; but I have not known of the existence of such fossils in 
any Museum. I have one, which is very small, from Mr. Strange’s 
collection. A similar shell is figured by Wolfart, Hist. Nat. Hass, 
inf. Tab. x. Fig. 1, 2. 
CIII. Hippopus. A subtransverse inequilateral shell : the hinge 
formed of two compressed entering teeth ; the crescent, or posterior 
depression, closed. 
This genus includes the Chama hippopus, Linn. I do not know of 
any fossil shell of this genus having been found. 
CIV. Cardita. An inequilateral bivalve. The hinge with two 
unequal teeth : the hinge tooth the shortest, beneath the beaks ; the 
other longitudinal, beneath the insertion of the cartilage. 
Chama calyculata, Linn. Cardita variegata, Brug. List. Tab. 34/, 
Fig. 84, may be taken as the type of this genus. 
These shells are irregular in their forms, and have two marks of 
attachment. 
