360 Memoirs of the Indian Museum. [Vol. V, 



the same handful of weed may possess any combination of the peculiarities men- 

 tioned. It is very probable that the inequality of the valves characteristic of Mr. 

 Preston's M. chilkaënsis is due to unequal pressure at an early stage of growth, the 

 crowded condition of the shells (pi. xvi, fig. 1) easily explaining how this may have 

 occurred. 



There seems to be very little difference between our specimens of M . undulata 

 and those described by Dunker and Reeve. The former author gives the length of 

 the shell as 11 lines (about 23 mm.) and our largest specimens are of exactly the 

 same size. Only a few individuals, however, attain these dimensions, the circum- 

 stances in which they live making it impossible for the majority of them to exist for 

 a prolonged period. A large number of those individuals that are attached to the 

 stems of Potamogeton must perish with that plant, which dies down in June or July, 

 though it is possible that some are able to transfer themselves to the roots, which of 

 course persist. In this position there is great danger of their being overwhelmed by 

 mud. Those individuals, on the other hand, that are attached to filamentous algae 

 growing on stones are mostly killed by desiccation in spring or early summer. 



We have no evidence, therefore, in the case of this species that its abnormal 

 environment in the Chilka Lake has produced anything of the nature of à racial 

 dwarfing or distortion. It is naturally a variable species, as is proved by the ap- 

 parent discrepancies in Dunker' s and Reeve's descriptions, both authors having 

 had before them specimens from the samelocality (the Moluccas) and collection. We 

 are not aware that the species has been recorded from any other Indian locality but 

 the Chilka L,ake , but we have specimens from Port Canning in the Gangetic delta. 



Modiola striatula, Hanley. 



(Plate XV, figs. 7-18 ; plate XVI, fig. 2.) 



1842-56. Modiola striatula, Hanley, Cat. Recent Bivalve Shells, p. 241, pi. xxiv, fig. 29. 



1858. Modiola striatula and emarginata (Benson MS.), Reeve, Conch. Icon., X, Modiola, pi. x, figs. 



72, 73- 

 1909. Brachyodontes emarginaius, Lynge, Danske Vid. Selsk. Skr. (7) not. og. math., V, p. 135. 



1909. Modiola cochinensis, Preston, Rec. Ind. Mus., Ill, p. 278, fig. 2. 



1910. Modiola jenkinsi, Preston, ibid., V, p. 36, fig. 5. 



1911. Modiola annandalei and celator, Preston, ibid., VI, pp. 40, 41, figs. 4, 5. 



1914. Modiola emarginata, Preston, ibid., X, p. 304. 



1915. Modiola taprobanensis, Preston, Ann. Mag. Nat. Hist. (8), XVI, p. 84, fig. 



1916. Modiola taprobanensis, Preston, Rec. Ind. Mus., XII, p. 35. 



The synonymy of this species presents great difficulties, owing, we are convinced, 

 to the extreme variability of the shell. Among the specimens from the Chilka Lake 

 Mr. Preston has recognized no less than four species, while in our more recent collec- 

 tions we find selected shells that agree precisely with his types of two others. We are 

 by no means certain that the synonymy we give is exhaustive, for it seems not at all 

 improbable that, when large series from estuarine tracts and lagoons in the Oriental 

 region are compared, it will be found that other forms at present regarded as distinct 

 fall well within the limits of variation of M. striatula. It is noteworthy, moreover, 



