110 Records of the Indian Museum. [Vol. XY, 



separation is then reflected forwards in an acute angle along the ventral 

 aspect of a triangular membrane attaching the dorsal margin of the 

 labial palps to the side of the visceral mass, and ultimately curves back- 

 wards round the anterior end of the inner and then the outer gill (the line 

 of curvature forming about | the circumference of a circle). It then 

 passes obliquely downwards and backwards nearly in a straight line 

 along the ventro-lateral aspect of the non-glandular portion of the 

 kidney. Behind the foot, a canal ^ intervenes between the non-glandular 

 portion of the kidney and the line of separation. In Laniellidens the 

 line is placed on the dorsal aspect of the canal, whereas in both Solenaia 

 and Physunio it lies on the outer side of the canal itself. Furthermore, 

 the line of attachment of the outer lamella of the outer gill is placed 

 below the line of separation in Lamellidens and Solenaia, but in Phy- 

 sunio in this line. This line is oblique and situated on the outer side of 

 the kidney ; it forms an acute angle with another line drawn from the 

 anterior end of the outer gill to the posterior end of the attachment of 

 the labial palps on their dorsal aspect in Physunio (more so in P. ferru- 

 gineus than in P. micropteroides), a right angle in Lamellidens marginalis 

 and an obtuse angle in Solenaia soleniformis. Lastly, the line of separa- 

 tion passes along the ventral margin of the posterior adductor muscle 

 to its posterior aspect, where it is joined to the opposite mantle-lobe 

 through the intervention of the gills. It then curves round the posterior 

 aspect of the posterior adductor muscle to join its fellow at the postero- 

 dorsal aspect of the muscle. 



The inhalent and exhalent apertures are bounded by the thick, 

 pigmented margins of the mantle-lobes. The inhalent aperture is twice 

 the exhalent aperture in height. The margin of the inhalent aperture is 

 beset with tentacles in all the genera here described ; in Lamellidens 

 and Solenaia the tentacles are short and stout and are nearly of the same 

 size, and are arranged in a single row, but in Physunio they are elongate 

 and conical and of three sizes which are arranged irregularly in three 

 rows, the largest ones (larger than those in Lamellidens and Solenaia) 

 being placed internally. 



The exhalent aperture is smooth in Solenaia and Physunio, but pro- 

 vided with a row of minute tubercles in its extreme lateral margins in 

 Lamellidens. The margins of the aperture extend further downwards 

 in Physunio than in Lamellidens and Solenaia. 



II. Labial palps. 



The lahial pal/ps, outer and inner, are continuous in front with the 

 anterior and posterior lips of the mouth ; they are elliptical in Lamelli- 

 dens and Solenaia, but a little more elongate in Physunio. The point at 

 which they are fused with the lips of the mouth forms a shallow con- 

 cavity on the ventral margin in Lamellidens and Solenaia, but presents a 

 slight flattening in Physunio. The labial palps are free from one another 

 except in three-fourths the length of the dorsal margin from the anterior 

 end ; they are connected with the body by a triangular membrane 

 attached to their fused dorsal margin. The apex of the membrane 



^ To be described witl) the kidney. 



