71 
PECTUNCULUS, Lamarck. 
Arca, Linn. 
Gen. Cuar. A bivalve subequilateral shell, hinge 
with many alternately inserted teeth placed in 
a single arched row. Cartilage of the hinge 
partly internal, attached to a flat triangular 
striated surface. 
PECTUNCULUS decussatus. 
TAB. XXVIL—Fig. 1. 
Spec. Cuar. Transversely obovate; sides rather 
straight; surface covered with numerous lon- 
gitudinal strie. Hinge teeth twenty-five to 
thirty. Margin thick, plain. 
AG 
Generaxry about half an inch wide, slightly depressed ; 
besides the lines of growth, and distinct longitudinal stria, 
which give the shell a beautiful appearance, there are many 
very close fine transverse striez, which can hardly be seen 
without a lense. 
Extremely common among the looser earth from the 
upper part of the clay stratum, thrown out from the 
excavation at Highgate, as alsa in the more dense py- 
ritaceous depressed nodules or septaria, often surrounding 
the edges in multitudes. The middle figure is from an 
extraordinary specimen presented to me by the indefat- 
igable friend to science, B. G. Snow, Esq. It was appa- 
rently a ball of Pyrites covered with these shells, lying in 
many directions, some filled with coloured Pyrites; but when 
broken at one end, it was found to be a rather irregular 
crust, containing within it a quantity of the new resinous 
substance in a state approaching decomposition, as it had 
lost its transparency, and is of an earthy aspect, looking 
like light brown dry cracked clay, but is readily inflamed. 
I am glad I had made a memorandum of it, as the Pyrites 
was in so forward a state of decomposition, that its falling 
to pieces could not be prevented, even under water. 
