I9I7-] J- HORNELi, : Indian species of Meretrix. 169 



retain their olive grey periostracum free of stain. These shells 

 grow uninterruptedly, show no rest phases, become stout in sub- 

 stance, and show no corrosion of the umbones ; not infrequently 

 two radial lines, sometimes of chevron-shaped marks, are apparent 

 in the umbonar region. The latter approximate to the coloura- 

 tion seen in young specimens of M. meretrix var. morphina, while 

 the undecorated ones approach Hanley's M. ovum. These forms 

 never seem to attain a larger size than 35 mm. in length, as they 

 either die off when the great floods come or else become stained 

 and corroded and pass thereafter as variations of the large form 

 described from the seaward channels. 



Further up the backwater, conditions become less favourable 

 for M. casta ; the bottom is less sandy and more muddy, the 

 action of the rapid flow of the river current becomes a new factor, 

 low salinity continues over a greater length of time, while, finally, 

 the appreciable amount of organic acids present in the drainage 

 from the hills and adjacent rice fields causes rapid corrosion of the 

 shell, especially upon and around the umbones. One of the most 

 marked effects of these altered conditions, and of the efforts made 

 to give accommodation to them, is a change in the form of the 

 shell. From being cordate and ventricose, the shape changes to 

 one which is distinctly compressed laterall}'^ and of pronounced 

 elongation in the antero-posterior axis. The young shells are 

 almost almond-shaped, so extreme is the compression and accom- 

 panying elongation. With age they become blunter at either end 

 and in their extreme phase exhibit a distinct convergence in out- 

 line and general shape to that of the freshwater mussel {Lamelli- 

 dens marginalis). The umbones show considerable modification 

 during these changes ; in typical M. casta they are set obliquely, 

 their apices directed forwards and inwards ; in the riverine forms, 

 now being noted, they are usuall}^ straightly incurved (c/. Hanley's 

 M. ovum — " natibus recte incurvatis "), and as their points usually 

 disappear through corrosion, this distinction becomes still more 

 emphasized ; even in cases also where the umbones originally 

 were curved forwards, this appearance becomes obscured and is 

 eventually lost by the effects of corrosion. 



So greatly specialised is this form that it appears worthy of 

 being given a varietal name, for the sake of clearness. As 

 Hanley's M . ovum occupies a fairly central position in the chain 

 of gradations to be grouped in the new variety, I propose to re- 

 tain ovum as a suitable name for this assemblage. I must say, 

 however, that no clear line of distinction can be drawn between 

 this and the more estuarine forms. Every gradation can be found 

 between both sections, and a complete series of examples can 

 easily be formed, showing every step in the change from a short 

 ventricose cordate form through a roundly ovate stage to an elon- 

 gated somewh.it laterally compressed elliptical form, totally unlike 

 that standing at the other end of the series. 



The substance of shells from west coast estuaries is distinctly 

 thinner than that of those from the east coast, due possibly to a 



