210 Records of the Indian Museum. [VoyL. XVIII, 
anal aperture and the formation of a siphonal orifice consequent 
on the development of the two siphons, the arrangement is different 
from that in the Unionidae. Anteriorly the two flaps are united 
with one another to a point a little below the anterior adductor 
muscle. From this point to an imaginary vertical line drawn in 
continuation of the anterior border of the posterior adductor 
muscle to the free edge of the mantle, the two flaps are separate, 
forming the large pedal orifice for the protrusion of the foot. At 
the posterior termination of the pedal orifice the two flaps are 
united by a well-developed muscular connection, thus separating 
off the pedal from the siphonal orifice. The siphonal orifice 
extends to a little below the upper margin of the posterior 
adductor muscle, and encloses the anal and the branchial siphons. 
The mantle in the siphonal region is notched a little below the 
middle, marking off the regions for the two siphons. Above the 
siphonal orifice the mantle flaps « are united with one another as on 
the antero-dorsal border. 
There are two adductor muscles. Of these the anterior one 
lies a little below the anterior margin of the pedal orifice, and 
is circular in outline and of about the same size as the posterior 
adductor. The latter lies just above the anal siphon. A small 
pedal retractor muscle, ovoidal in outline, is situated above the 
posterior adductor muscle; its fibres, which by their contraction 
retract the foot as the shell-valves close, are easily traceable to 
the foot. The well-developed radiating palleal muscle-fibres of the 
mantle have been referred to already; they originate near the 
palleal line and end slightly behind the free edge of the mantle. 
In consequence of the absence of a distinct siphonal sinus on the 
shell the siphonal contractors are feebly developed and appear as 
specialized radial palleal fibres, which are more numerous in this 
region, are specially thickened and have a distinct antero-posterior 
course. 
In all the preserved specimens the siphons are fully contracted. 
It is not possible, therefore, to decide as to their respective 
lengths. The two siphons are, however, quite separate from one 
another, the upper or anal siphon being the smaller of the two. 
It has a rounded orifice with one or two small papillae surround- 
ing it, and has the anus opening into it anteriorly just behind the 
posterior adductor muscle. The lower or branchial siphon is much 
larger, with an ovoidal orifice in the contracted condition, and 
bears three to four rows of elongate papillae on its external 
orifice. 
The attachments of the gills are quite normal. The outer 
lamellae of the outer pair are attached to the mantle, the inner 
lamellae of the outer pair to the outer lamellae of the inner, while 
the inner lamellae of the inner pair are attached along a little 
more than the anterior half to the abdominal mass, the rest 
becoming fused with the same part of the lamella of the gill of 
the opposite side. The outer pair of gills are much shorter in 
both length and width than the inner pair. 
