Invertebrate Gallery of the Indian Museum. 141 



occur in beds in the mud of the Bay of Bengal at depths 

 of two to five hundred fathoms. 



2nd Family Trigoniacea, [Case 172C]. 



This is a geologically ancient but now small and almost 

 extinct Family, Both the animals and the shells much re- 

 semble Cockles. The edges of the mantle-lobes are not 

 united, and there are no siphons. The interior of the 

 shell is remarkable for its magnificent pearly lustre. The 

 hinge-teeth are few and large. One of the finest species 

 of the whole family is Verticordia eburnea dredged in the 

 Andaman Sea at a depth of 188 — 220 fathoms, and described 

 and figured in the Annals and Magazine of Natural His- 

 tory for December 189!. A perfect specimen of this 

 beautiful species is exhibited in Case i j2C. By some 

 mischance a duplicate of this species found its way in 1893 

 into the London market and has been redescribed under 

 the name of Verticordia optima. 



3rd Family Unionacea, [Case 173 A, B]. 



This large but uniform family includes the various 

 Freshwater Mussels, which are found in rivers lakes and 

 ponds in every part of the world. The lobes of the mantle 

 are not united below, but their fringed and bilobed pos- 

 terior edges — as may be noticed in the several dissections 

 of Unio above Case 173 — do cohere slightly below the level 

 of the vent so as to mark off two distinct channels, — an 

 incurrent and an excurrent,. The shell is covered on the 

 outside with a thick dark epidermis, and on the inside is 

 almost as lustrous as the Pearl-Oyster. Freshwater Mus- 

 sels, in fact, not unseldom secrete very good pearls. 



4th Family Luclnacea, [Case 173 B,C]. 



In this not very large but very widely distributed family 

 the shell, typically, is stout and almost circular in outline. 

 The condition of the mantle-lobes varies : sometimes 

 {Astarte) they are separate throughout ; sometimes 



