OE SOUTHERN INDIA. 297 



VII. Order,— UNIONACEA. 



I will class in this order all the fresh-water Pelecypoda which are generally 

 known under the name of Unio, besides those few remarkable, partially adherent, 

 species, called ^theria, and its allies. All these shells agree with each other 

 in many important characters, they being always covered with an olivaceous 

 more or less thickened epidermis, pearly inside, with an external, or partially sub- 

 internal ligament, with two muscular scars (only in Millleria the anterior becomes 

 obsolete with advanced age), and an entire pallial sinus. Generally there is a small 

 supplementary muscular scar behind or below the anterior one, and sometimes 

 partially confluent with it ; it is produced by a particular muscle which I shall 

 describe in the Unionid^. The hinge has either no teeth at all, or only a set of 

 anterior teeth, or there are anterior and posterior teeth present. These hinge-teeth 

 are always of a peculiar construction, and I think the term '' lateral teeth," as used 

 by Jeffreys in his British Conchology (vol. i, p. 28,) is more correctly applicable to 

 them than the term cardinals, which generally is reserved only for the anterior 

 teeth. There is, however, a free interspace in the place of the true cardinals to be 

 observed between the two series of teeth, and consequently the anterior teeth are 

 quite equivalent to the posterior, and both should be called laterals. This espe- 

 cially becomes apparent when we compare forms like Castalia and similar genera. 



The animals of the UNION A CEA mostly are closely allied to those of the 

 AsTARTiDJE, All have a widely disunited mantle, but some possess a single anal 

 opening, this form including apparently the typical section; others are said to 

 possess two siphonal openings, and again some other externally allied forms have 

 no special siphonal openings. Thus we find a repetition of the same variations 

 which I noticed in the L TJGINA CEA regarding this point. There are two pairs of 

 gills and two of palps in all genera; the foot is generally sub-trigonal, wedge- 

 shaped, rarely elongated and thickened at the end. 



Eollowing H. and A. Adams' arrangement in this group of Pelecypoda, I shall 

 retain his two families TInionidjE and Mutelid^ ; in the first the animal possessing 

 only one, in the second two siphons. If this distinction in the animals exists the 

 classification has good ground, whatever astonishingly absurd Deshayes may find 

 in it ; but I do not wish to be answerable for it, as I had no opportunity to examine 

 any of the animals of the second family. Besides these two I shall treat in this 

 order also the JEtheeiidm. Only the first family has as yet some palseontological 

 importance for us. 



XXX, Family— UNIONIDJE, 



I take as an example of the organisation of this family the following descrip- 

 tion from two of our common Bengal species, JI, marginalise Lam., and Z7. cceru^ 

 leus. Lea. Body oval, with a large laterally compressed foot, obtusely sharpened 

 at the edge and more or less produced in front, strongly extensible, mantle entire, 

 disunited all round with only a small anal opening, being separated by a very 

 thin commissure from the inhalant portion ; the edges of the latter are strongly 



