MY ABM. 161 



widely, at the two extremities. Some of the species grow 

 to a very large size. The valves are united by a hinge of 

 variable character. The ligament is in some genera ex- 

 ternal, in others internal. The animal has its mantle closed 

 in front, excejit for the passage of a foot, which is seldom 

 developed in proportion to the mass of the body. The 

 siphons are greatly prolonged, and united almost to their 

 extremities ; their orifices are fringed. Both body and 

 shell are often invested in a coarse and wrinkled epidermis. 

 All the Mollusks of this tribe bury themselves in sand, 

 gravel, or mud. They are palatable articles of food, and 

 are much sought after in many places. 



The evidence of geology would go to shew that this 

 family was much more developed in the earlier epochs of 

 the world than now, since, during the Jurassic period 

 especially, many more species of Myadce^ mostly belonging 

 to peculiar generic groups, lived than are now known to 

 exist. 



Not a few of the extinct forms, even of those oldest in 

 time, bear a striking resemblance to their living allies, and 

 are with difficulty discriminated from one another ; a dif- 

 ficulty which is increased by the state in which these 

 fossils are found, being most usually only external casts, so 

 that in a majority of instances the structure of the hinge 

 cannot be perceived. The general similarity of the mem- 

 bers of this family depends on the slight amount of varia- 

 tion of shape and sculpture of surface presented by their 

 shells. Professor Agassiz has attempted to group them 

 into many genera, chiefly founded on the modifications of 

 the latter character, but these are too slight and uncertain 

 to permit of the adoption of such sections with safety 

 either by the naturalist or geologist. 



Judging from their associates, the Myadae, at the epoch 



VOL. I. Y 



