OF SOUTHERN INDIA. 91 



small number will have to be considerably enlarged when the animals of some 

 other genera have been examined. 



I would here particularly draw attention to the genus dementia, Gray, 1840, 

 (see H. and A. Adams' Genera, ii, p. 433). The shells in general character and 

 in the dentition of the hinge very much resemble some Glauconomycs, and there 

 appears to be a kind of a wdiite cartilage mass situated in a deep open groove 

 behind the beaks, covered up by the ligament which extends backwards. The 

 pallial sinus of dementia is quite similar to that of Dosinia, and accordino^ to the 

 statement of II. and A. Adams the animals of both would also seem to be similar ; 

 the genus must, therefore, be for the present retained in the sub-family dosiniinje, 

 I am sorry to say that I have not yet had an opportunity of observing the animals of 

 dementia, but the character of the shell with its pale olivaceous epidermis is cer- 

 tainly nothing like what w^e are accustomed to see among dosiniinje {vide postea). 



I do not know the typical species of Naranio, Gray, but from the description 

 of the species N. rubiginosa (Proc. Zool. Soc, 1863, p. 425, pi. 37, fig. 17), it 

 would appear that the genus is closely allied to dementia; H. and A. Adams 

 {'' Genera") consider, however, Naranio as identical with dwristodon. And lastly, 

 I may mention here the genus Blainvillia of Hupe, of which I already spoke in 

 the Mactmibjs (see p. 54). 



1. Glaucoiiomya, Bronn, 1838, (Glauconome, Gray, non idem, Goldfuss, 

 Defrance, &c.). Shell subtruncate or somewhat attenuated behind, slightly 

 inequivalve and inequilateral; hinge composed of three oblique teeth in each 

 valve, those of the right generally larger than those of the left, the last bifid tooth 

 is more oblique than the others. Margin behind the beak thickened, adapted for 

 the support of a long external ligament, the posterior end of wdiich is attached 

 internally at the end of the nymphae ; pallial sinus narrow and long, but generally 

 not extending as far as the beaks ; surface of shell covered with a green imbricated 

 epidermis. 



There are about 18 recent species known of this genus, all estuary or littoral. 



2. Tanysiphon, Benson, 1858. Shell small, oval, subtruncate behind, covered, 

 like that of Glauconomya, with a dull greenish epidermis ; pallial sinus broad and 

 extending as far as, and sometimes beyond, the region of the umbones ; hinge with 

 three teeth in each valve, almost perpendicular, the anterior two in the left valve 

 smaller, the last being in both the largest, short, and broad, more resembling a 

 cartilage process of the Myiidm than a simple tooth ; ligament external very short, 

 its posterior end attached internally behind the cartilage. The external form of 

 Tanysiphon is the same as that of Glanconomya, but the hinge differs in the direc- 

 tion of the teeth and the form of the cartilage processes, &c., &c. 



There is as yet only a single species, T, rivalis, Benson, known, wdiich was 

 found near Calcutta in a canal (Tolly 's-nuUah) subject to the influence of tide, the 

 water being generally a little brackish. The species is common here; but very 

 rarely occurs in other places of the Sandarbans. I met wdth another species on 

 the Arracan coast and found also some imperfect valves at Penang. 



