fossils, under the name hypocephaloidcs, a representation of the calcedqnic 

 cast, from Tilsbury, is given Plate XII. Fig. 12. 



One circumstance respecting the shells of this genus demands parti- 

 cular mention. Lamarck has figured, in the recent shell T. margaritacea^ 

 two marks of attachment, one at the bottom of each tooth of the hinge; 

 but I have not been able, in any of the ibssil 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, Tig. 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. 



CII. Tridacna. A subtransverse inequildteral shell : the hinge formed 

 of two compressed and entering teeth ; the crescent, or posterior de 

 pression, 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 considerable 

 size; but I have not known of the existence of such lossils in uny Mu- 

 seum. I have one, which is very small, from Mr. Smm^e's collec- 

 tion. A similar shell is figured by Wolfart, Hist. Nat. II ass. 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 

 eeth: the hinge tooth the shortest, beneath the beaks; the other longi- 

 tudinal, beneath the insertion of the cartilage. 



Chama calyculata, Linn. Cardita variegata, Brug. List. Tab. 347, /. 84, 

 *nay be taken as the type of this genus. 



These shells are irregular in their forms, and have two marks of 

 attachment. 



