1S35.] three New Species of Fresh water Bivalves. 453 



the boundaries of the genera of the Naiada, can hardly justify its 

 adoption. The assumption that genera are separated in nature by an 

 hiatus has been ably combated by the zoologists of our present Eng- 

 lish school. Genera melt into each other, and the circumstance of 

 the flanking individuals of each cohort being in contact does not 

 militate against their grouping round the standards which form their 

 rallying points. Mr. Lea has named this shell from a character which 

 exists only in young specimens, and which is also observable in a 

 distinct and interesting species (U. Theca, Mini), of which I possess 

 an unique example from the river Cane in Bundelkhand. The largest 

 specimen of Unio bilineatus in my possession, is in breadth, 2.3 

 inches. The adult shell has a brown epidermis inclining to fulvous 

 towards the basal margin, and occasionally the anterior side inclines 

 to form a wing as well as the posterior. Mr. Lea gives as a charac- 

 ter, cardinal teeth " double in the right valve only;" but in every spe- 

 cimen which I posses?, a thin lamina parallel with the principal lobe 

 of the cardinal tooth, is more or less developed in the left valve, and 

 interlocks with those on the right ; and it is this double lamina in 

 the left valve which forms one of the most valuable distinctions be- 

 tween the adult bilineatus and the occasionally symphynotous young 

 of Unio marginalis, which has no trace of a double lamina in the left 

 valves. The concentric undulations, on the beaks, which are also 

 observable in the young of U. marginalis, also disappear in the adult 

 bilineatus. — W. H. B. 



Unio Olivarius. Plate XVI. fig. 38, of ditto. 



" Testa ovatd, transversa inflatd, pellucidd ; valvulis pertenuibus ; natibus 

 prominulis ; epidermide pertenui, Irevi et olivce colorem habente ; dentibus car- 

 dinalibus magnis laminatis erectisque lateralibus laminatis brevibusque ; mar- 

 garild pertenui albdque. 



" Shell ovate, transverse, inflated, pellucid ; valves very thin ; beaks 

 slightly elevated ; epidermis olive, very thin and smooth. Cardinal teeth 

 large, erect and lamelliform j lateral teeth short and lamelliform ; nacre 

 very thin, white and pearly. 



Hab. Burrill river, India, Dr. Burrough. 

 Diam. -7, Length *8, Breadth 1-5 inches. 



" Shell ovate, transverse, inequilateral, inflated, pellucid : substance of 

 the shell very thin ; beaks slightly elevated, rounded and devoid of un- 

 dulations : ligament very small : epidermis olive, very thin and smooth : 

 rays obscure, cardinal teeth large, erect and lamelliform ; lateral teeth 

 short and lamelliform : anterior cicatrices slightly confluent : posterior 

 cicatrices confluent : dorsal cicatrices not perceptible ; cavity of the 

 beaks wide ; nacre very thin and bluish white. 



" Remarks. — This interesting little shell is from the fine collection made 

 by Dr. Burrough, during his travels in India, and I am indebted to his 

 3 N 



