TELLINA. 287 



oblong, the posterior one widest ; pal Hal impression with a 

 deep and wide sinus. 



Animal ovate, compressed, mantle entirely open in front, 

 its margin fimbriated ; siphons long, separate throughout, 

 usually nearly equal, their orifices plain, or very indis- 

 tinctly toothed. Foot large, triangular, compressed, api- 

 culate. Labial palpi large, lanceolate. Branchial leaflets 

 united in pairs on each side. 



The mollusks of this genus live in sand or sandy mud, 

 buried beneath the surface ; the majority at low water- 

 mark, or in considerable depths. A few species have a 

 range as deep as the coralline zone. The number of known 

 kinds of TellirKe exceeds two hundred. They occur in all 

 seas, but more than a third of them are inhabitants of the 

 Indian Ocean. Many species are found also in the West 

 Indian region, and on the west coast of South America. 

 They increase in numbers from the poles to the equator. 

 The arctic forms are squalid and unattractive ; those inha- 

 biting the tropics, on the contrary, gaily-coloured and orna- 

 mental, so as to be sought after for decorative purposes. 

 Their range in time is not so certainly known, though 

 several Palaeozoic and secondary shells have been assigned 

 to this generic group. In the upper part of the cretaceous 

 system, and in beds of the tertiary (especially Eocene) 

 epoch they are distinctly present. 



The structure of the branchial and the labial palpi, with 

 the simple character of the extremities of the tubes, dis- 

 tinguish the animals of this genus from those of Psammo- 

 bia, whilst the shells may usually be distinguished with 

 facility by their umbonal folds and dentition. Nevertheless 

 the two genera are exceedingly closely allied. 



