Gallery 
VIII. 
120 GUIDE TO THE FOSSIL INVEETEBRATE ANIMALS. 
have endured with little change from the .Silurian Epoch to 
the ]n’esent day. 
1^ • LAMELLIBRANCHIAj of which the oyster and 
cockle are examples, usually have complicated gills (l)ranchiae) 
formed of many lamellae or plates ; the foot is rarely used 
for crawling, hut is gener.dly wedge-shaped, whence they are 
also called PELECYPODA (hatchet-foot). Tliese features 
are naturally associated with a sedentary habit of life and 
with the suppression of the head-region. The shell is not 
deposited by a visceral hump, hut by two flaps of the 
mantle, placed on the right and left sides ; hence it consists 
of two valves, which are joined along one edge by a ligament 
and generally a hinge, and can be closed by powerful 
muscles (adductors). The lamellibranchs are confined to the 
water, and most arc mai’ine. Some, like the oyster, aie 
fixed ; most burrow in mud or sand, and a few l)ore into 
wood or rock. This Class, as a v\’hole, presents a somewhat 
uniform structure, and it can hardly be said that any of the 
numerous attempts to divide it into Orders has met with 
general acceptance. Therefore we shall only indicate some 
of the chief variations that can be seen in the shell. These 
are ; (1) the adductor muscle-scars, whether two equal, 
two unequal, or only one ; (2) the outline of the mantle- 
attachment (Fig. 68, 2), whether simple or indented by a 
sinus due to certain muscles that work tubular extensions 
of the mantle called siphons (but the absence of a sinus does 
not imply the absence of siphons) ; (3) perfect or deficient 
symmetry of the shell-valves ; (4) shell-structure, whether 
porcellanous or nacreous; (5) the arrangement of the 
ligament ; (6) the hinge (Fig. 69, (j), whether plain or 
toothed, ami the varying numbers and development of the 
hinge-teeth. 
A few ill-preserved shells, apparently of simple Lamelli- 
branchs, have been found in the Cambrian rocks of AVales, 
Thuringia, and North America. In Ordovician rocks thev 
are still rare, but in Silurian times a score of families 
existed, mostly with thin shells of simple type. The 
Devonian saw the beginning of brackish and fresh-water 
lamellibranchs ; these increased in Carboniferous times, when 
also appeared Allorisma, the first form known to have a 
retractile siphon. With the Trias many of the older genera 
disappeared and new families came in, followed by others 
in the Jurassic period, when also Trigonia arose and soon 
flourished in numbers (Plate V.). Among Cretaceous lamelli- 
