GINGLYMACONCHA. NUCULAD. 281 
liquely vertical tooth entering between two unequal teeth in the 
right valve. 
The respiratory tubes separate; the lower one very long, 
the upper much shorter. 
TRIGONELLA LISTERIANA. 
Shell very compressed, with elevated concentric strize, which 
are often antiquated. Length two inches two eighths ; height 
one inch five-eighths. Colour yellowish or croceous, some- 
times whitish, but generally tinged with black, from the mud 
in which it resides. 
Lister, Anim. Angl. t. iv. f. 23. 
Mactra Listeri, Gmel. Syst. i. 3261; M. & R. Trans. Linn. 
Soc. vil. 71; Turt. Conch. Dict. 83. 
Venus Borealis, Penn. Brit. Zool. iv. 96. 
Trigonella plana, Da Costa, Brit. Shells, 200. t. xiu. f. 1. 
Tellina plana, Donov. Brit. Shells, t. lxiv. f. 1. 
Mactra compressa, Mont. Test. Brit. 96. 
Ligula compressa, Mont. Test. Brit. Supp. 23; Flem. Edinb. 
Encycl. vii. 88. 
Lutraria depressa, Lam. Hist. Nat. des Anim. sans Vert. v. 
469. 
Mactra piperata, Dill. Desc. Cat. 142. 
Inhabits mud at the mouths of rivers, estuaries, or arms of 
the sea, and is very common. 
The umbones are slightly tinged with black bronze in some 
specimens, but such are of rare occurrence in Britain. 
FAMILY VIII. NUCULAD. 
Shell equivalve, closed, somewhat triangular or elongate. 
Umbones before the middle of the shell. Hinge under the 
umbones with a vertical process bearing the ligament, with 
many teeth before and behind the umbones. Ligament in- 
ternal. 
The respiratory tubes not united together. 
