202 
F. H a a s ; Beiträge zu einer Monographie der asiatischen Unioniden. 
This species is allied to the last one fmicropteroides), but larger, with a thinner shell, with 
the wing better developed, with the diverging longitudinal ridges stronger, with the striae on 
the external surface finer and more regular, with the epidermis of a rich iron-brown (some- 
times with darker concentric lines), with the nacre paler, with the lateral teeth more prominent 
and the pseudo-cardinal teeth of a different shape. In general appearance the young shell 
resembles P. semialatus. 
The shape is variable, I figure on plate 19 a series showing different growth-stages as well 
as variation in old shells, 
, Measurements of shells (in millimetres) 
Type 
Length of shell 71 73 70 
Greatest depth of shel 47 49 50 
Thickness of shell 18 18 20 
Type-specimen, M, 11290/2, Zoological Survey of India (Ind, Mus,), 
Habitat, — This mollusc is very abundant in the semi-liquid mud at the bottom of the 
central region of the Inlé Lake in water from 7 to 12 feet deep. Its position of rest in the mud 
is with the longer axis vertically upright and the valves buried nearly as far as the posterior 
end of the wing. It progresses through the mud by thrusting out the foot downwards and for- 
wards, This forces the whole animal in the opposite direction and at the same time presses the 
dorsal edge of the valves downwards. The foot is then withdrawn and the shell regains its 
vertical attitude a little in advance of its former position. The foot is thrust out again and the 
movement repeated. The result is that the mollusc progresses with a see-saw motion, dorsal edge 
in front, the wing assisting greatly in overcoming friction by acting as a kind of ploughshare. 
The posterior end of the valves is often covered with a massive brown alge of the family Rivu- 
lariaceae, Annandale, 
Die Merkmale, die nach Annandale diese Art von seinem Phys, micropteroides unter- 
scheiden sollen, sind so geringfügig, daß ich in ihr nicht mehr als die Seeform von jenem er- 
blicken kann, 
Schlußbemerkungen über die systematische Stellung von Physunio. 
Nach Prashad's Angaben stimmt das Glochidium von Physunio ziemlich gut zu dem, das 
ich (Conch, Cab,, S, 224, Taf, 26, Fig, 5) von Rectidens prolongatas Dr, beschrieben habe. Doch 
die beiden Gattungen einander zu nähern, ist wohl nicht angängig, da bei Rectidens außer der 
ganzen äußeren auch noch die ganze vordere Hälfte der inneren Kieme marsupial wird, während 
das Marsupium bei Physunio auf die äußere Kieme (gravidus) oder nur die Mitte der äußeren 
Kieme (micropteroides und ferrugineus) beschränkt ist. 
