164 MYTILIDiE. 



ligament linear, internal. Three muscular impressions. 

 Pallial impression obscure. 



Animal oblong, its mantle closed except a passage for 

 the foot and the two siphonal orifices. Branchial opening 

 prolonged into a tube with a circular fringed orifice ; anal 

 opening plain and subsessile. Foot short, ligulate, with a 

 conspicuous byssal groove. 



This small group of Mytilida; is important as constituting 

 a link between that family and the Unionidee. and like the 

 latter inhabits fresh water. It was founded by Van Bene- 

 den in 1835, for the Mytilus polymorphic of Pallas, and in 

 the " Annales des Sciences Naturelles " for that year, the 

 Belgian naturalist has published a very excellent and de- 

 tailed monograph upon it, including an anatomy of the 

 animal, the peculiar features of which warrant a separation 

 from Mytilus, not clearly indicated by the shell. As long 

 ago as 1824, Mr J. de C. Sowerby called attention to 

 characters, and in 1826 the arrangement of the mantle and 

 siphons was noticed by Von Bar, in Oken's " Isis. r> In 

 the same year with Van Beneden the genus Tichogonia 

 was constituted for it, as a shell only however, by Ross- 

 massler. Its claims to a generic rank were remarked by 

 Gray in 1825. There are several species both recent and 

 fossil. The living ones have a wide dispersion, and geolo- 

 gically the genus dates as far back as the epoch of the 

 Weal den. 



The fossil genera Congeria and Enocephalus appear to 

 be identical with Dreissena. A number of extinct species 

 are recorded from tertiary strata of the parent region of the 

 species we have to describe. 



