ON THE DEVELOPMENT OF LINGULA ANATINA. 43 



superficial layer of the shell becomes bright green (PL VI., Fig. 

 84., and PI. VIL, Fig. 97.). The colors do not dissolve out in 

 alcohol or in any other reagent. 



The Protegulum remains as such in the shell. In the dorsal 

 valve it is faintly visible, while in the ventral it is always out- 

 lined by a definite ridge. On transverse section the lateral 

 margins of the Protegulum slightly project from the general 

 surface of the shell. Brooks on the contrary, observed the 

 Protegulum on the dorsal valve only ('78 p. 39.). 



The shell is of medium thickness throughout, thinning, 

 however, toward the margins. Near the hinge line it thickens 

 with age. Here the ventral valve (vt. sh.) (PI. VIL, Fig. 97.) 

 ends in a thin lamella, which is the remnant of the early con- 

 necting cuticula. This lamella bends over dorsally so as to 

 overlap, and lock with the thickened edge of the dorsal valve. 

 The latter thickens a little anterior to the hinge line where it 

 thins out and is provided with a horizontal ridge directed outward 

 (PI. VIL, Fig. 97., r.). 



Referring now to the extremities of the hinge line in surface 

 view (PI. VIL, Fig. 96.), the hinge line of the ventral valve 

 is somewhat longer than that of the dorsal, as Brooks states 

 ('78 p. 38). While the ventral valve (vt. sa.) terminates at either 

 end with a pointed tooth, whose sharp edge forms part of the 

 margin of the Protegulum (prt.), the dorsal valve (dr. sh.) spreads 

 out into a thin plate whose margins are rounded and sinuate. 



The secretion of the shell goes on uniformly all over the 

 dorsal and ventral walls of the body proper as well as on the outer 

 surface of the mantle lobes, but the most active organ of secre- 

 tion is the mantle margin. This fact is best confirmed in the 

 posterior mantle where the growth is early arrested on account 



