1835.] three New Species of Fresh -water Bivalves. 451 



" Shell narrow-elliptical, transverse, inequilateral, subcylindrical; valves 

 thin ; beaks rather elevated, rounded and undulated ; cardinal teeth 

 lamelliform and double in the right valve only ; lateral teeth straight ; 

 nacre bluish white, pearly and iridescent. 



Hab. River Hoogly, Hindostan, G. W. Bxakik. 

 Diam. -6, Length -8, Breadth T6 inches. 



" Shell narrow-elliptical, transverse, subcylindrical, disposed to be 

 6traight on the sides and basal margin ; substance of the shell thin ; beaks 

 near the anterior margin rounded, somewhat elevated, and corrugated with 

 diverging undulations; ligament rather short and straight; epidermis 

 finely wrinkled and bluish green, particularly on the posterior part ; rays 

 very indistinct ; posterior slope furnished with small undulations and 

 two irregular rays on each side ; cardinal teeth lamelliform and double in 

 the right valve only ; lateral teeth straight and lamelliform * anterior 

 cicatrices distinct ; posterior cicatrices confluent ; dox-sal cicatrices within 

 the cavity of the beaks ; cavity of the beaks wide and rounded ; nacre 

 bluish white, very pearly and iridescent. 



" Remarks. — This species was brought from Calcutta by Mr. Blakie, 

 to whose kindness I am indebted for it and many other fine shells. As far 

 as I have been able to ascertain, it has not been described. From the 

 roughness of the beaks it might perhaps be thought to be only a variety 

 of corrugata (Lam.). On comparing the two species, however, they will 

 be found to be entirely distinct ; the corrugata being " ovato-rhombea," 

 while the cceruleus is " angusto-elliptica." In some specimens the nacre 

 is slightly rose-coloured along the basal margin." 



Note. — This shell is extremely common in tanks in the vicinity of 

 Calcutta, and is met with in profusion in the Ganges, Jumna, and 

 their branches. The epidermis is ordinarily brown, and I have only 

 met with the dark-green variety figured and described by Mr. Lea in 

 jhils in Bundelkhand. It varies much in diameter, being sometimes 

 extremely ventricose, while another frequent variety is remarkable 

 for the smallness of its diameter. All the varieties may be at once 

 referred to this species, by attending to the generally polygonal out- 

 line of the posterior part of the shell, and to the rugse on the beaks 

 and slopes, which radiating in two different sets, form by this junction 

 on the back of the shell, in fine specimens, a series of acute angles. 

 The shallow variety above-mentioned, which has a paler green epi- 

 dermis, and which is somewhat alate posteriorly, shews this cha- 

 racter, in the greater perfection. The extreme varieties would be 

 regarded as distinct species, if alone presented for examination ; but 

 I possess a beautiful series which connects them so gradually as to 

 leave no doubt of their identity as a species. My largest specimen, 

 which is an odd valve, is 1"95 inches in breadth. The nacre is occa- 

 sionally tinged with salmon colour. — W. H. B. 



