On the opening and closing of Bivalves. 219 



adductor muscles, the cartilage is always compressed, and when 

 it is situated ia a shelly cavity, all allow it to be so ; and it is 

 exactly the same when it is walled in by an inelastic ligament on 

 one side, and pressed upon on the other by the callus ; thus there 

 is always the same endeavour to regain its situation and open the 

 valves, let it be either external or internal. The only shells that 

 appear to be deficient in this cartilage, are the Myastropha of ^ 



my arrangement of MoUuscu, where its place is supplied^by an 

 abductor muscle. 



The cartilage, which is very peculiar for its black colour and 

 pearly lustre, and for its great elasticity when fresh and moist, 

 appears to be the nerve (Nervus) of Aldrovandus, when he is 

 speaking of the Spondylus, and he appears to have had a very good 

 idea of the use and structure of this part. When this substance 

 is dry it is very brilliant, and has a fine display of colours; and I 

 am informed that it is used by the jewellers for black opal, but 

 it is apt to crack; by what name they call it I am not aware, 

 which I much regret, as being of such a peculiar structure and use, 

 it is worthy of a distinct appellation. 



§ II. On the Opening and Closing of Bivalves. 



From what I remarked in the last observations, the valves of the 

 bivalve shells must constantly have a tendency to be open or sepa- 

 rate from one another; now, these valves are closed by the ad- 

 ductor muscles of the animal, which are generally considered as 

 the only agents employed for the purpose of counteracting the 

 elastic cartilage ; but it is a well known fact that (voluntary) 

 muscular action is only a periodical power, which requires rest by 

 relaxation, before it can be called again into action, and that it 

 would be very painful were it to be continually on the stretch, as 

 any person may prove on himself by trying to keep the arm ex- 

 tended, or any part of the body confined in a particular position 

 for a length of lime. This eflect is counteracted by a peculiar 

 clastic ligament, similar to the Paxwax, Whit Leather, or Liga- 

 rnentum Nuchoi the herbivorous quadrupeds, which is formed for 



