112 Records of the Indian Museum. [Vol. XV, 



lying in contact with the upper end of the anterior aspect of the adductor 

 muscle. In Solenaia also, it is triangular, but lies in contact with 

 the notch in the posterior adductor muscle. In Physunio again it is 

 triangular, but is placed above and in front of the dorsal aspect of the 

 adductor muscle. 



(6) The elevator muscles are arranged in two patches one on each 

 side, on the dorso-lateral aspect of the visceral mass. 



In Lmnellidens marqinalis each patch is placed at the junction of 

 the anterior one-third and posterior two-thirds of the body length and 

 consists of very short columns of muscles 5 — 7 or more in number and 

 arranged irregularly ; they are at once inserted into that portion of the 

 valve lying in front of the umbonal cavity. In Solenaia they are placed 

 behind the region of the umbo. In Physunio they are placed above and 

 behind the anterior adductor muscle far forward in position as compared 

 with those of Lamellidens or Solenaia. 



IV. Gills. 



The gills are more or less similar in Lamellidens and Physunio. They 

 are elongated and placed obliquely, and are approximately 4 times as 

 long as broad. The inner gill is wider and longer, and extends a little 

 more forward than the outer one. The outer gill is narrow and pointed 

 in front, narrow and tapering behind. The inner gill is wider than the 

 outer and extends below the outer gill. In Lamellidens the outer gill 

 is nearly of the same width all through except at the tapering posterior 

 and rather abruptly narrowed anterior end. The inner gill projects 

 downwards beneath the outer one more in the anterior two-thirds of its 

 length than in its posterior one-third. In Physunio the outer gill is 

 comparatively narrow, being widest near the middle of its length and 

 tapering at both ends ; it presents a notch in the ventral margin near 

 the anterior end ; the inner gill projects downwards most in front, 

 slightly behind and least in the middle. 



In Solenaia the gills are much narrowed and elongated, being about 

 ten times or more as long as they are broad ; the inner gill extends a 

 little more forward than the outer, as in the other species, and projects 

 a little below the outer gill equally in its entire length except at the 

 anterior end ; the anterior and posterior ends of the outer gills are 

 narrower and more pointed than those of the inner gill. 



Attachments of the cills : — 



(1) The outer lamella of the outer gill is attached to the inner surface 

 of the mantle-lobe. In Lamellidens the line of attachment lies a little 

 below the separation of the mantle-lobes from the body-wall, so that the 

 outer wall of the outer suprabranchial chamber is formed by a portion 

 of the mantle-lobe ; the case is similar in Solenaia. In Physunio, how- 

 ever, the line of attachment lies at the line of separation of the mantle- 

 lobe from the body-wall. The attached margin of the outer lamella 

 is placed on a higher level than that of the inner lamella in Lamellidens 

 and Physunio, but at a lower level (or at least in the same level) in 

 Solenaia. 



(2) The inner lamrlln of the outer gill is attached to the side of the 

 visceral mass, at its anterior end, tlu; ventro-lateral aspect of the non- 



