116 LECTURES ON MOLLUSCA. 



America, Corrugaria and Iridea; to Africa, Ccelatura; to Asia, Naia, 

 Lanceolaria, Dipsas, Hyriopsis, Nodularia; to Australia, Hyridella, 

 Parreysia, and Cucumaria. The European Mysca has but slight pecu- 

 liarities. 



In the Margaritana group, without lateral teeth, the old pearl 

 muscle, M. margaritifera, is found throughout the colder regions of both 

 Old and New World. It used to be extensively fished in the British 

 islands for the occasional pearls. Complanaria, Alasmodonta, Leptodea, 

 and Strophitus are all found in North America; Monocondylcea and 

 Plagiodon in South America ; and Monodontina is an Asiatic form. 



The Anodons of Europe, though very variable in form, are believed 

 to belong to one species; but in North America the distinct forms are 

 very numerous. The young of many Unionids are known to attach 

 themselves by a byssus at pleasure; but in the South American Bysso- 

 donta this appears to be permanent. An accurate arrangement of the 

 family, founded both on peculiarities in the animals and on geograph- 

 ical distribution, is still a great desideratum. 



Family MYCETOPIDJE. 



In the South American Mycetopus, the mantle is open except around 

 the anal aperture ; the shell resembles a toothless Solecurtus ; and the 

 foot is very much lengthened, ending in a hammer-shaped knob. 



Family IKIDINID^). 



The shells in this family closely resemble those of the Unionids ; but 

 the animals differ in having the mantle-flaps united at the side to form 

 two short pipes. Castalia is like the Arciform Unios, with the hinge- 

 teeth furrowed, as in Corbicula. Hyria has spreading wings like 

 Metaptera or Avicula, with the teeth somewhat plaited. Leila can 

 scarcely be distinguished from Anodon by the shell alone. These forms 

 are peculiar to South America. In Africa are found Pleiodon, with 

 the hinge line broken across into numerous teeth, like Area; Callisca- 

 plia, with slight crenulations on the hinge line; Spatha, with a bent 

 hinge, like Alasmodonta; and Iridina, like a very trans verse Anodon. 

 There are no members of this family known from the northern con- 

 tinents. 



Family ETHERIAD^I. (Fresh-ivater Oysters.) 



Just as the CJiamas might be regarded as Cockles turning into 

 oysters, the Etherids may be considered as Anodons making even a 

 greater stride in the same direction. The shells of Ethernet, which 

 were first discovered by Bruce, being eaten in the Upper Nile, are free 

 when young, and shaped like Anodon; they have then probably a foot. 

 But when adult, they are attached and irregular, resembling an olive- 

 green oyster with two muscular scars. There is then no foot, and the 

 mantle is freely open. It is found in the tropical rivers of Africa and 

 South America. 



Still more remarkable is the Mulleria from New Granada. It begins 

 life, free, like the Etheria, with two adductor muscles; but when adult 



