1918.] Ekendranatii Giiosii : Anatomy of Unionidae. Ill 



is directed forwards and the base forms a free margin behind ; the side 

 fused with the wall of the visceral mass is overlapped by a small portion 

 of the ventral margin of the inner gill. 



III. Muscles. 



(1) The anterior adductor muscle is irregularly triangular in shape in 

 Lamellidens and Physunio, and rather obliquely pyriform in Solenaia. 

 The relative position of the muscle and the pseudocardinal tooth or teeth 

 in the intact animal varies with the different species described. In 

 Lamellidens the anterior end of the tooth or teeth is placed just above 

 the middle of the attachment of the anterior adductor muscle, while in 

 Physunio it extends to the antero-dorsal angle of the muscle with a gap 

 between the two. 



In all the species described here tlie muscle is divided into an upper 

 and lower portion by an oblique septum extending between the two 

 valves. 



(2) The posterior adductor muscle is triangular (with the apex above 

 and the angles rounded) in Lamellidens and Physunio. In Solenaia it 

 is elongated horizontally, with nearly parallel upper and lower borders 

 and rounded ends ; in the anterior and upper corner there is a notch to 

 receive the posterior end of the muscle. 



(3) The anterior retractor muscle of the foot arises from the base of 

 the foot on its side, just behind the mouth ; it passes obliquely upwards 

 and forwards beneath the posterior lip of the mouth and on the inner 

 side of the lower portion of the protractor muscle to be inserted into the 

 valve just behind the anterior adductor muscle. In Lamellidens the 

 surface of attachment lies along the lower two-thirds or more of the 

 posterior margin of the adductor muscle, being widest below and taper- 

 ing above. In Solenaia it is placed behind the upper third of the pos- 

 terior margin of the adductor muscle, and is nearly circular in shape. 

 Lastly, in Physunio it is elongately triangular with the apex below, and 

 is placed behind the upper half of the posterior margin of the anterior 

 adductor muscle. 



(4) The protractor muscle of the foot arises on each side from the 

 visceral mass above the base of the muscular foot. The fibres radiate 

 from an obliquely placed triangular area and are collected into a very 

 short column, which passes obliquely downwards and forwards to be 

 inserted into the valve near the anterior adductor muscle. In Lamel- 

 lidens marginalis the muscle is placed below^ and behind the adductor 

 at a little distance from it ; in Solenaia it is placed behind and a little 

 below the anterior retractor, at a distance behind its ventral aspect. 

 In Physunio it is placed behind the adductor at the level of its lower 

 end and also at a distance from it. 



(5) The posterior refractor muscle of the foot on each side arises from 

 the extreme posterior and dorsal aspect of the visceral mass ; the two 

 muscles lie side by side in the middle line, pass obliquely upwards and 

 backwards between the two kidneys (at their posterior portions) and 

 are inserted into the valves above the level of the upper border of the 

 kidneys and in front of the posterior adductor nuisclc. In Lamellidens 

 marginalis the surface of attachment is more or less triangular in shape, 



