had long, retractile siphons, and was a burrower in the mud. " All bivalve 

 shells with this kind of siphon," said the Professor, " have a pallial sinus 

 in the shell, while those with short, non-retractile siphons are without 

 a sinus, as is the fresh-water clam. 



"In the fresh -water clam the dorsal part of the two shells are 

 fastened together by a ligament, and the valves are aided in opening 

 and closing, by a set of cardinal and lateral teeth. In this soft-shelled 

 clam, however, no such apparatus is present; but in its place there 



is in the left valve a broad 

 shelf extending from the 

 hinge, on which there is 

 a long tooth, and a tough 

 piece of brown cartilage 

 called the resilium. In the 

 right valve there is a de- 

 pression and a heavy ridge, 

 which fits into a corre- 

 sponding depression in the 

 left valve. This resilium 

 forms a cushion which is 

 constantly pushing the 

 valves apart. 



" It will be remembered 



Interior of right valve of clam, aa, anterior adductor 

 muscle scar; pa, posterior adductor muscle scar; pi, 

 pallial line; ps, pallial sinus; r, resilium, or so-called 

 cartilage; t, tooth-like enlargements of the hinge corre- 

 sponding to the teeth of the fresh-water clam; u, umbo. 



that iii the fresh -water 

 clam, the tough ligament pulled the two valves apart by contraction; 

 but in the soft clam the elastic cushion is constantly pushing the 

 valves apart. In both cases, the adductor muscles have to overcome 

 the strain to keep the valves together. The resilium is not a true 

 counterpart or analogue of the ligament, but it is a part of the latter 

 which has become separated from the true ligament, which is reduced 

 to a mere trace in this clam, because of its burrowing habits. 



" The interior of this shell is not lustrous and pearly, as in the fresh- 

 water clam, but is white like porcelain. For this reason the shell is 

 called porcellanous. The outside of the shell is very much roughened 

 by many concentric lines or ridges, which show where the animal 

 added to the size of its shell as it grew larger. The heavier ridges 

 indicate rest periods and by counting them we are able to tell the 

 age of the clam, as you observed in the fresh-water clams." 



Having thus explained to us the functions of the shell, the Professor 

 picked up a large shell from a pile lying near, carefully severed the 

 two adductor muscles by cutting them close to the shell with a flat- 



