CONCH1FERA. 273 



gona, Goldf. Isocardia axiniformis. Ph. Anodontopsis securiformis, Anatina 

 attenuata and Doldbra securiformis, M'Coy, probably belong to this genus. 

 Dolalra equilateralis, Amphidesma subtruncatum, Anodontopsis angusti- 

 frons, M'Coy, with many others from the Palaeozoic rocks, may constitute a 

 distinct genus, but their generic character has yet to be discovered. 



LYRODESMA, Conrad, 1841. 



Type, L. plan a, New York. Syn. Actinodonta, Phil. 

 Shell trigonia-shaped, rather elongated, with a striated posterior area; 

 hinge with several (5 9) radiating teeth, striated across; ligament external. 

 Fossil, 3 sp. L. Silurian : Canada, U. States, Brit. 



FAMILY VI. UNIONS. Naides. 



Shell usually regular, equivalve, closed ; structure nacreous, with a very 

 thin prismatic-cellular layer beneath the epidermis; epidermis thick and 

 dark ; ligament external, large and prominent ; margins even ; anterior hinge- 

 teeth thick and striated, posterior laminar, sometimes wanting; adductor 

 scars deeply impressed ; pedal scars 3, distinct, 2 behind the anterior ad- 

 ductor, one in front of the posterior. 



Animal with the mantle-margins united between the siphonal orifices and, 

 rarely, in front of the branchial opening; anal orifice plain, branchial fringed ; 

 foot very large, tongue-shaped, compressed, byssiferous in the fry ; gills elon- 

 gated, sub-equal, united posteriorly to each other and to the mantle, but not 

 to the body ; palpi moderate, laterally attached, striated inside : lips plain. 

 Sexes distinct. 



The river-mussels are found in the ponds and streams of all parts of the 

 world. In Europe the species are few, though specimens are abundant ; in 

 . America both species and individuals abound. All the remarkable generic 

 forms are peculiar to S. America and Africa. Two of these are fixed, and 

 irregular when adult, and have been placed with the chamas and oysters by 

 the admirers of artificial systems ; fortunately, however, M. D'Orbigny has 

 ascertained that the Mulleria, which is fixed and mono-myary when adult, is 

 locomotive and di-myary when young ! * 



Like other fresh-water shells, the na'ids are often extensively eroded by 

 the carbonic acid dissolved in the water they inhabit (p. 41).f This condi- 

 tion of the umbones is conspicuous in the great fossil Uniones of the Wealden, 



* In the synopsis at p. 252 it will be seen that each of the principal groups of 

 bivalves contains members which are fixed and irregular, and others which are byssi- 

 ferous, or burrowing, or locomotive. 



f Probably many of the organic acids, produced by the decay of vegetable matter, 

 assist in the process. It has been suggested that sulphuric acid may sometimes be 

 set free in river-water, by the decomposition of iron-pyrites in the banks : but Prof. 

 Boye of Philadelphia states that it has not been detected in any river of the United 

 States, where the phenomenon of erosion is most notorious. 



