FAMILY 5. MACTRACEA. 67 



utraque valva ; cum foveola angusta elongata interjacente. Liga- 

 mentum duplex, partim tenue et externum, partim breve et inter- 

 num, in foveolis cardinalibus insertum. Impressio muscularis pallii 

 sinu magno. 



The genus Amphidesma includes a numerous and well-defined group 

 of Mactracea : their shells are somewhat variable in form, some being 

 transverse, others orbicular, but are nevertheless readily distinguished by 

 the peculiar position of the ligament, which is duplex. There was at 

 first much confusion in the formation of this genus by Lamarck ; but by 

 the removal of some of his Amphidesmata to the Mesodesmata, we are 

 enabled to retain a very natural and interesting series. Many new spe- 

 cies too have been added since his time, contributing much to the im- 

 portance of the genus. 



The shell of Amphidesma may be described as being somewhat inequi- 

 lateral, transverse, nearly oval or round, and slightly gaping at the sides. 

 The hinge is composed of either one or two cardinal teeth in each valve, 

 with a long narrow pit lying between them for the insertion of the liga- 

 ment, which is duplex, partly external and thin, partly internal and 

 short. The palleal impression has a large sinus. 



Examples. 

 PI. XLVII. Fig. 1. 

 Amphidesma roseum, Sowerby, Proceedings Zool. Soc, 1832, p. 199. 



PL XLVII. Fig. 2 and 2*. 

 Amphidesma pulchrum, Sowerby, Proceedings Zool. Soc, 1832, p. 57. 



PL XLVII. Fig. 3. 

 Amphidesma pallidum, Sowerby, Proceedings Zool. Soc, 1832, p. 199. 



PL XLVII. Fig. 4. 



Amphidesma australe, Sowerby, Proceedings Zool. Soc, 1832, p. 200. 



k2 



