78 
NATURAL HISTORY. 
[EAST. ZOGL, 
striated. The Amphidesmte only differ from the TeUince in having the 
cartilage placed in an internal pit, and Cumingia only differ from 
the latter in the hinge-pit being projected into the cavity of the valves. 
The Petricolce are irregular shells which live in holes in stones, and 
have large irregular triangular teeth. The genus Donax is peculiar 
for having the hinder side of the shell much shorter than the front, 
giving it a truncated wedge-shaped form, which has caused most con- 
chologists to mistake the front for the back of these shells. The genera 
Hecuba, Cuneus, Latona and Iphiginia, differ from one another 
in the disposition of the hinge teeth. The Capsa are like Donaces , 
but more swollen, and covered with a thin olive periostraca. The 
Galathece are triangular shells with large hinge teeth, and covered with 
a hard thick periostraca, like the Cyrence ; w T hen young their teeth are 
like the Capsce, but as the shell enlarges in size and increases in thickness, 
the teeth increase in size, and become subdivided into separate lobes; 
they are only found in the rivers of Africa. 
The order of Cladopoda, on the contrary, have a large club-shaped 
foot, which is sometimes truncated and expanded at the end. The 
mantle-lobes are generally united, forming a bag open only in front to 
allow* of the passage of the foot. The syphons are large, exserted, 
and generally united together nearly to the end, and the gills are 
produced into the upper or anal syphon. The periostraca is extended 
so as to cover the exposed part of the mantle and syphons. 
The family of Pholad^e have the valves united only by a very thin 
ligament, scarcely thicker than the periostraca, in which are generally 
imbedded one or more shelly plates. In the Pholades, the place of the 
cartilage is supplied by a muscle situated on a reflection of the inner 
surface of the shell, over the umbones. They live in holes, which 
they form in the substance of shells, calcareous rock, wood, and resin. 
The foot is truncated in front, to enable them to retain their position 
w r hile they rotate their shell to form the holes, the substance in w’hich 
they burrow being partially softened beforehand by the juices of the 
animal. Pkolas has an elongated shell with three pieces on the back. 
The Parnia has only one piece, and the Zirfcea are short shells with 
very large gapes at each end, and no distinct dorsal pieces. / The 
Martesia , when the] animal arrives at full size, closes up the gape in 
the front of the shell with a shelly plate, and the dorsal ligament is 
covered with a large shield-like convex plate. The Talona differ from 
the latter in being longer, and in the back margin being reflected, and 
only furnished with two small back pieces. The Xylotrya are small 
globular shells like Teredines, but have two small dorsal pieces. The 
Teredince are only found in a fossil state, and are very like the Mar- 
tesice ; the valves are very short, subglobular, with a lengthened tube 
behind, but they are longer. The Teredo, instead of having any plates 
in the dorsal ligament, continues to bore deeper and deeper into the 
wood, and lines the holes as he proceeds with a shelly tube for his pro¬ 
tection ; and they have at the end of then body small calcareous pallets 
which close the mouth of the tube. In Teredo these pallets are simple 
and battledore-shaped. In Pankia they are elongated, and formed of 
small cones one within the other, looking somewhat like a quill. 
Lamarck compared these processes to the legs of Cirripedes. The 
Kuphus has ovate pallets toothed at the tip; the tubular case of 
