92 THE NAUTILUS. 
collection of the Philadelphia Acad. of Nat. Sciences ; also, that the 
writer, who has collected many scores of specimens, has never found 
them associated with any other species of the genus. 
JOHN Forp. 
Philadelphia, Pa., Dec. 12th, 1889. 
THE SHELL-BEARING MOLLUSCA OF RHODE ISLAND. 
BY HORACE F. CARPENTER. 
FAMILY UNIONID. 
This family formerly called Naides, embraces those shells com- 
monly called fresh-water clams or mussels. They are found in 
rivers and ponds all over the world, but they reach their greatest 
perfection in this country. Over twelve hundred species are known 
to science, named and properly classified, of which more than half 
inhabit the U.S. This family has been made the special study of 
the late Dr. Isaac Lea, of Philadelphia, who died at the advanced 
age of ninety-five years, on the eighth of Dec., 1886. He devoted 
fifty years of his life to the study of the Unionide and has described 
more new species than all other conchologists together. He has read 
before scientific and other societies one hundred and ‘tifty-seven 
papers and has been honored by degrees and honorary membership 
of twenty-five of the most prominent universities and scientific 
associations of the world. His great work, “ Observations on the 
genus Unio” consists of thirteen quarto volumes, illustrated by 
hundreds of beautiful plates. 
The shells of this family are not in general very attractive on the 
outside, but the interior of the valves are always lined with a beau- 
tiful pearly substance called nacre, which in some specimens are 
pure white and in others salmon, rose-red, blue, green, purple, ete. 
The sexes in this family are distinct, which is an exception to the 
rule in a large majority of the species of Mollusca, where the sexes 
are united in each individual. The shells exhibit but little varia- 
tion in form except the usual one, that the females are more ventri- 
cose and broader behind than the males. 
The animals of this family are all capable of producing pearls, 
some of which are of great beauty and value. In one instance six- 
teen pearls were obtained from a single specimen. One of the 
