THE BIVALVES 3383 



produces. Some of the rivers of Scotland, and the Conway in North Wales, 

 have always had a great reputation for their pearl fisheries. Although not equal 

 in lustre to the Oriental jewel many of these river pearls are beautiful. Unlike 

 Unio, the Dreissensiidce, as represented by Dreissensia, have the mantle margins 

 united ventrally, with an anterior opening for the slender foot and byssus, and 

 prolonged posteriorly into two siphons. The shell is shaped like the common 

 marine mussel. The European D. polymorpha was first noticed in England about 

 seventy years ago, and is supposed to have been introduced attached to Russian 

 timber. 



SUBORDER Tellinacea 



This group includes the families Tellinidce, Scrobiculariidce , Donacidce, Mactridce, 

 Mesodesmatidce, and Cardiliidce , the first of which is the most extensive, and con- 

 tains the most beautiful forms. Here the animal is remarkable for the great 

 length of the slender separated siphons, the fringed mantle margins, and large 

 labial palpi and foot. The shells are nearly always compressed, subequivalve, 

 joined by an external ligament, and furnished with hinge teeth. The scars 

 of the adductors are far apart, and the pallial impression is generally very deeply 

 sinuated. None of the bivalves are more beautifully colored than the tellens, the 

 prevailing tints being purple red, crimson, and various shades of yellow. The 

 surface sculpture of the valves is often beautiful and delicate. Tellens live in 

 sand or mud at slight depths in every sea, and the species may be counted in 

 hundreds. Nine species of Tellina are British. The Scrobiculariidce form a smaller 

 and less showy family than the preceding, the shells being united by an internal 

 ligament, and sometimes by an external one also. The soft parts are like those of 

 the Tellinidce. Some species of the genus A bra have been dredged at enormous 

 depths, both in the Atlantic and Pacific, but the majority of the family have been 

 obtained in comparatively shallow water. The British Scrobicularia piperata buries 

 itself in the mud of estuaries, and can extend its siphons five or six times the 

 length of its shell. Although in some respects the wedge shells {Donacidce) agree 

 with the Tellinidce, they differ as regards the gills. In Donax the shells are of a 

 triangular or wedge shape, and have the inner margin of the valves crenulated. 

 They are united by an external ligament, and furnished with cardinal and some- 

 times lateral teeth. The wedge shells live buried in the sand in shallow water 

 in warm regions all over the world, and three species occur on the British coasts. 

 The genus Iphigenia, which has no lateral teeth, inhabits estuaries on the coast of 

 Africa, Brazil, the West Indies, and Central America. In the mactras (Mactridce} 

 the shells are often more or less triangular, and have an internal ligament, 

 the siphons being united the entire length, and fringed at the ends. About 

 one hundred and fifty species of Mactra are known. They occur on sandy 

 shores in most parts of the world at shallow depths, six being British. This 

 family comprises a large number of genera and subgenera, mainly .distinguished by 

 modifications of the hinges; Mulinia, Spisula, Standella, Rangia, Rceta, and 

 Eastonia being the more important. The remaining families, Mesodesmatidce and 

 Cardiliidce, are not of special importance, and may be passed without further reference. 



