NATURAL HISTORY. 
[EAST. ZOOL. 
84 
Pectens have nearly equal valves and unequal ears. The Janira have 
one valve very concave and the other flat. The Neithea are like the 
Pectens , but there are several transverse teeth on the hinge margin. 
The Amusia have both the valves nearly flat, smooth externally, and with 
ribs within. The Plagiostoma only differ from the Lima in having no 
or very little gape in the front margin, and the Limaculce are nearly 
equilateral. The Pedums are thin flattened shells -with a large tri¬ 
angular area, marked with the long cartilage groove; they live in 
corals, and are often distorted. 
The family of Spondylidce have a small foot without any byssus, and 
the shells are attached to rocks and stones by the outer surface of one 
of the valves. In Spondylus and Plicatula, the hinge margin is pro¬ 
vided with two large interlocking teeth in each valve, and in Hinnites 
the hinge is toothless like the Pectens , with which it has generally been 
confounded. In Spondylus the teeth are thick and roundish; in Plica¬ 
tula long, compressed, and transversely grooved on the sides. The 
fossil genera of Podopsis and Dianchora are peculiar for having the in¬ 
ternal opaque coat of the valves usually destroyed during the process of 
fossilization ; frcm the want of this coat, they have frequently been 
mistaken for Terebratulce. 
The animals of the remaining families have lamellar shells and no 
foot. 
The family of Oysters ( Ostreid^e) have a thick laminated shell, and 
the animal has short lips, separate from the gills; they live attached, 
like the Spondyli, by the outer surface of their shells; the cartilage is 
placed in a large triangular internal pit. The Ostrcea are irregular 
laminar shells, the lower valve being concave and the other flat. The 
Gryphaea chiefly differ from them in the apex of the lower valve being 
produced incurved, so that the upper valve looks like an operculum to 
the cavity of the other. They are only attached by the tip of the lower 
valve. JExogyra, like the Gryphcece , have the tip of the lower valve 
incurved, but being attached by the greater part of the outer surface of 
this valve they greatly resemble Chama in appearance. The Plec- 
ironia have nearly equal valves, which are strongly plaited dn the 
margin ; they often attach themselves to branches of trees growing in 
the sea, and assume the form of the bodies to which they happen to be 
attached. 
The family of Placunid^e are very peculiar for having a very com¬ 
pressed body and thin nearly transparent shells ; the cartilages are 
placed on the edge of two diverging ridges on one of the valves, which 
flt into two grooves in the other. These shells are sometimes used as 
glass to glaze windows. 
The family of Anomiad^e have the thin pellucid shell of the former, 
but the body is usually rather more convex, and they are attached to 
marine bodies by a peculiar muscle, which passes out through a notch 
in front of one of the valves; this muscle after a time secretes on the 
surface to which it is affixed a stony substance, formed of longitudinal 
shelly plates, probably deposited between the fibres of the muscles, 
which has been called a stopper, and by some considered as a third 
valve. In Anomia this stopper is free. In Placunanomia it is fixed in 
the notch, which is obliquely prolonged as the shell enlarges. These 
shells become gradually moulded to the surface they rest on. Thus, if 
