126 GUIDE TO THE FOSSIL INVEKTEBRATE ANIMALS. 



Gallery have endured with little change from the Silurian Epoch to 

 the present day. 



IV. LAMELLIBRANCHIA, of which the oyster and 

 cockle are examples, usually have complicated gills (branchiae) 

 formed of many lamellae or plates ; the foot is rarely used 

 for crawling, but is generally wedge-shaped, whence they are 

 also called PELECYPODA (hatchet-foot). These 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, but 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 are marine. Some, like the oyster, are 

 fixed ; most burrow in mud or sand, and a few bore into 

 wood or rock. This Class, as a whole, 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, g), whether plain or 

 toothed, and 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 Wales, 

 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- 



