174 



UNIONID^. 



Fig. 476. 



U. ochraceus. 



and encloses a broad, depressed space, with the margins compressed 

 into a keel ; base regularly rounded. Surface toleral)ly regular ; 

 epidermis lying in fine loose folds about the posterior end, color 



olivaceous, rather yellowish 

 in shells of the ordinary size, 

 and finely radiated and zoned 

 with dark olive over every 

 part of the shell. Interior a 

 very delicate rose-color, or 

 deep salmon-color, tinted with 

 rose-red. Cardinal teeth com- 

 pressed, striated, directed for- 

 wards, and nearly parallel with 

 the hinge-margin ; lateral teeth 

 short; cavity of the beaks capacious. Length, two and three fourths 

 inches ; height, two inches ; breadth, one and one fourth inches. 



Some specimens are found much larger. I have one from Penn- 

 sylvania which measures four inches, two and two fifths inches, 

 one and three fourths inches. Such shells become quite thick and 

 proportionally elongated posteriorly ; they lose the radiations of the 

 epidermis, which is of a dark olive-color. I do not know that any 

 such shells have been found in Massachusetts. 



This shell is very rare, and I do not know of its having been 

 found anywhere except in Plymouth ponds. When young it is 

 scarcely to be distinguished from U. ccmosiis ; but it is more in- 

 flated, and the radiations of the epidermis are finer, and cover more 

 of the shell ; nor is it so glossy, and its interior is more colored. 



Ociiu«» MARGARITAIVA, Schumacher. 1817. 



Shell transverse, inequilateral ; hinge like that of Unio, except 

 that it is destitute of a lateral tooth. 



Margaritana arcuata. 



Fig. 75. 



Shell more or less kidney-shaped, very inequilateral, thick, beaks not promi- 

 nent; epidermis pitchy-black; Avithin bluish- white ; teeth erect, conical, grooved. 



Alasmodonla arcuata, Barnks, Silliman's Journ. vi. 277, pi. 12, fig. 20 (1823). — ADAiMS, 

 Shells of Vermont, in Thomps. Hist. 165. — Gould, Inv. Mass. 1st ed. 113, tig. 75. 

 -De Kay, Nat. Hist. New York, 197, pi. 14, fig. 224. 



