46 PALEONTOLOGY 



these structures mark the dorsal region, not the posterior, 

 as in brachiopods. The region where the valves separate 

 most widely when the shell opens is therefore ventral 

 (instead of anterior). The measurement from the dorsal 

 to the ventral edge is the height of the shell, the measure- 

 ment at right angles to this (in the plane separating the 

 valves) is the length, being from the anterior end to the 

 posterior; and the measurement across both valves, at 

 right angles to both height and length, is the thickness. 



Lamellibranchia are very rare in the oldest strata ; they 

 were rarer than Brachiopoda throughout the Palaeozoic 

 era ; as the latter diminished in numbers, they increased, 

 until in the Cainozoic era and at the present time they far 

 outnumber the Brachiopoda. They are less restricted 

 in their distribution, a few genera being found in fresh 

 waters, though the great majority are marine. 



Though the mode of life is similar to that of brachio- 

 pods, they are more free to move about. A few fix them- 

 selves after the larval stage by cementation (as the 

 oysters) or by silky threads (byssus), as the mussels, never 

 by a muscular pedicle ; many burrow in sand or mud, 

 a few bore into harder materials, the majority move 

 sluggishly about the bottom, and a very few (as the 

 scallops) swim by a series of jumps. 



In the majority of cases, the animal is bilaterally 

 symmetrical, and the right and left valves are as nearly 

 counterparts as is possible, seeing that the hinge-teeth of 

 the one must come opposite the sockets of the other 

 (equivalve shells). But in some families, especially those 

 with a fixed habit, the valve on which the animal lies is 



