1919.] N. ANNANDALE & B. PrasHAD: Mollusca. 59 
Family UNIONIDAE. 
Genus Lamellidens, Simpson. 
1900. Lamellidens, Simpson, Proc. Nat. Mus.(Washington), XXII, p. 854. 
1911. Lamellidens, Ortmann, Nautilus, XXIV, p, 106 
1918. Lamellidens, Prashad, Rec. /nd. Mus. XV, p- 145. 
1919. Lamellidens, id., ibid., XVI, p. 293, fig. 4. 
The occurrence of this genus in Seistan proves the existence 
of a distinct Indian element in the fauna. 
Lamellidens marginalis, Lamarck. 
Subsp. rhadinaeus, nov. 
(Pl.ifi, figs; 9,10; plowilietes..7 Pr.) 
Shells from Seistan only differ from those of the forma typica 
from Bengal in a few particulars, but the differences are constant 
in a large series. In shape the shell is somewhat variable, but is 
close to that of the var. corrianus, Lea, being more transverse and 
having the upper margin straighter than that of the forma typica. 
In this respect it is intermediate between the two varieties. It is 
slightly more tumid than either and in old shells one or other of 
the valves is as a rule bent outwards slightly at the point at which 
the foot emerges, causing the shell to gape at this point. The 
sculpture of the young shells is finer and sharper than in the com- 
mon Indian varieties. The teeth of the hinge are more prominent 
than in either form, the pseudocardinals stouter and the laterals 
more bent. In their stoutness the teeth approach those of the spe- 
cies or variety L. jenkinsonianus (Benson) of Bengal and Assam. 
The older part of the shell is pale cream-colour or silver-grey 
externally. This usually deepens, as the shell grows, to chestnut, 
and sometimes even to black. Some shells, however, are pale 
greenish; there is usually at least one pale yellowish zone on the 
darker region, and the margin is sometimes pale. The nacre is 
bluish white and has a china-like lustre with very little iridescence. 
Two phases can be distinguished, the difference being visible 
even in very young shells. 
Phase A.—The shell is of considerable size and thickness, 
with the growth-lines strongly developed, the epidermis of the 
lower part dark and the hinge-teeth unusually stout and prominent, 
approaching those of some species of Unio in development. 
Phase B.—The shell is smaller, thinner, less inflated and paler 
in colour and the hinge-teeth are less stout and not so prominent. 
The young shell seems to be slightly more elongate in phase B 
than in phase A and is distinctly smoother in the former. It does 
not differ in essential characters from that of the typical form of 
the species, but has the sculpture less well developed ; it is rather 
shorter in proportion than that of the var. corrianus. Its sculp- 
ture is very well preserved even in old shells, the glochidial shell 
often remaining as a minute tubercle. When about 25 mm. long 
the shell has a fairly prominent dorsal “‘ wing” and is thin, almost 
translucent and of a greyish colour. 
