igar.] B. Prashad : Swnatraii Molluscs. 501 



Genus Nerita, Linn. 



This genus is represented by two widely distributed species . 

 'N . lineata and .V. planospira. 



Nerita lineata, Cliemn. 



1897. Nerita lineata, v. Martens, u/>. cif., |i 2ig. 



A large series of specimens of all ages collected from the 

 mangrove swamps at Belawan (Deli), and a few from the Soengei 

 Belawan (Deli) not far from the sea, are represented in the coUec- 

 tiim. Some of the empty shells contain hermit-crabs. 



Nerita lineata, Anton. 



1S97. Nerita planospira, v. Martens, op. cif., p. 2ig. 



The specimens of this species were collected along with those 

 of the preceding species in the same mangrove-swamps. They 

 are of all ages and some have hermit-crabs in the empty shells. 



Genus Septaria, Fer. 

 Septaria tessellata (Lam.). 



1899. Septaria tessellata witli vars. clypeolnm, compressa and lineata. 

 Dautzenberg, op. cit., pp. 23-26, pi. i, figs. 14, 1 ia, 15, 16. 



Manjr specimens of the forma typica, showing all grades of tes- 

 sellated colouration and closely corresponding with Lamarck's 

 original figures and also with those of Dautzenberg cited abovo, 

 are represented from the streamlet along the road to Anak Laut, 

 Sabang. Besides these specimens of the forma typica, shells of the 

 three varieties clvpcolum (Recluz), compressa (v. Martens) and 

 lineata (Lam.), collected in the mangrove-swamps at Belawan 

 (Deli) are also present in the Sumatran collection. 



Family OstreidaE. 



Genus Ostrea Linn. 



Specimens of three species of this genus are represented in the 

 collection from the regions of mangrove-swamps in Sumatra. 

 One of these, which I consider to l)e identical with the widely 

 distributed miocene and recent Ostrea gryphoides (Schlotheim),_ be- 

 longs to the subgenus Ostrea, s.s., and was probably brought into 

 the estuarine region by the tides as there are remains of corals on 

 the shells,' but it is likely that the species in Sumatra, like an 

 allied form found living in the Chilka Lake,' is a true inhabitant 

 of brackish waters. The other two species belong to the subgenus 

 Alectryonia, and are true estuarine forms. 



' Might it not be possible that the shells are subfossil, and at the presimt day 

 are found in the mangrove-swamps in consequence of the retiring of the sea by 

 land elevation, which is here very prominent, fden Hoop]. 



■! Sec Annandale and Kemp, .Mem. hid. Miis. V, pp. 348, 349 (1916). 



