This arrangement is such that the large adductor muscle is 

 pretty well inclosed, there being only a small postero-ventral 

 portion of the muscle that is not surrounded by the other organs. 



The portion near the hinge consists almost entirely of the 

 digestive gland, commonly called the liver, with the oesophagus, 

 stomach and first part of the intestine inclosed in it. During 

 the season of reproduction, a thin layer of gonads extends over 

 its surfaces laterally as well as anteriorly and posteriorly. The 

 portion ventral to the foot consists almost entirely of the gonads 

 with the coils of the intestines running through them. The foot 

 is largely muscle with a rather extensive byssal gland inclosed 

 in it. The liver region extends from valve to valve of the shell 

 and is covered closely with a very thin portion of the mantle. 

 The reproductive portion and foot are comparatively narrow and 

 are suspended between the gills, being supported in large part 

 by the adductor muscle. These portions do not occupy a posi- 

 tion midway between the two valves but are nearer the right 

 than the left valve. 



The foot is a comparatively slender, roughly cylindrical organ 

 somewhat larger where it is attached to the body than elsewhere, 

 cleft at its free end, so it may be spread to form a " sole " as in 

 Nucula and Yoldia, and showing the large opening of the byssal 

 gland to the right of the middle line, on the ventral surface some- 

 what behind the sole (fig. 8). 



The foot has lost its symmetry, being twisted so the ventral 

 surface is directed somewhat toward the right valve. It seems 

 to be in a large measure a degenerate organ that is practically of 

 no service as an organ of locomotion. The animal depends 

 upon swimming by clapping the shell valves together to change 

 position. The foot may however be greatly extended and 

 thrust between the valves of the shell. When protruded it leaves 

 the shell just ventral to the notch at the base of the anterior 

 ear, and may be moved from place to place. The flaps on the 

 sides of the foot are moveable and are frequently separated 

 somewhat but there is no such active movement as in Yoldia 

 (6) or other forms with this type of foot, with which I am 

 acquainted. In the species under consideration I have never 

 seen the foot protrude far out of the shell and have never seen 

 the animal attach itself with a byssus. 



2 17 



