2l6 AN INTRODUCTION TO THE STUDY OF FOSSILS 



grows forward, again extending the shell and leaving behind as 

 a thickened line the deposit of shelly material formed during 

 the quiescent stage. The shell thus bears a succession of 

 raised lines of growth or varices (Fig. 91), indicating inactive 

 periods and separated by thinner bands marking periods of 

 active expanding growth of the mantle. There may be many 

 such growth lines in a year, as favorable and unfavorable condi- 

 tions alternate, minor oscillations in conditions as that between 

 high and low tide, day and night, etc., being indicated by very 

 fine growth lines. 



The dorsal or cardinal margin of each valve is strengthened 

 by a thick vertical plate, the hinge plate. Upon this are strong 

 projections and ridges forming a system of teeth and sockets, 

 the teeth of one valve fitting into the sockets of the other. 

 This interlocking apparatus constitutes the hinge of the shell ; 

 by means of this an exact closure of the valves necessarily oc- 

 curs. The teeth are of two kinds, the anterior or cardinal, 

 consisting of short transverse elevations, and the posterior or 

 lateral, longer teeth parallel with the dorsal margin of the valve. 



The rounded, elevated portion of each valve is called the 

 umbo; it ends in a point, the beak, which curves toward the 

 anterior end of the shell. This is the oldest portion of the shell 

 and from it grow^th has proceeded. Hence the beak is the initial 

 point of the concentric growth lines. Under the beak and an- 

 terior to it is a depression called the liinule. 



The sexes are distinct. The reproductive elements are ex- 

 truded into the mantle chamber and out into the surrounding 

 water through the exhalant siphon. Millions of eggs and 

 spermatozoa are produced, most of which perish, since it is 

 only by chance that their union is effected and fertilization thus 

 results. 



Development starts directly in the fertilized egg ; this results 

 in the formation of a blastula which soon invaginates, thus 

 passing into the gastrula stage. Thus, about ten hours after 

 the egg has been fertilized, the embryo has become a spherical 



