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 Ford. 

 Philadelphia, Pa., Dec. 12th, 1889. 



THE SHELL-BEARING MOLLUSCA OF RHODE ISLAND. 



BY HOPwVCK 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 Unionidse and has described 

 more new species than all other conchologists together. He has read 

 before scientific and other societies one hundred and rifty-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 fiimily 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, etc. 

 The sexes in this family are distinct, which is an exception to the 

 rule in a large majority of the sj)ecies of MoUusca, 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 



